With Hiccup and Jack being the only two males in the group, the two are usually portrayed to be close companions, almost like brothers. Since Jack Frost (Jokul Frosti) comes from Nordic myths and Hiccup is a Viking, fans believe it could have been possible that Hiccup believed in Jack, and therefore been able to see him. Also, it is said that on Hiccup's native island of Berk it 'snows nine months of the year and hails the other three."' Though this could be a bit of an exaggeration, it is a fact that the island has some cold weather patterns, which could be a result of frequent visits from Jack. Jack and Hiccup's personalities can also clash, as Jack's freewheeling and somewhat irresponsible nature can clash with Hiccup's more responsible and thoughtful personality, causing the "voice of reason" to rein him in. Jack is also quite mischievousness, so he would play pranks on Hiccup frequently.
In the fandom, their friendship can go beyond a platonic relationship. "Hijack" (or "Frostcup", as some fans call it) originates to a time before RotBTD when Rise of the Guardians and How to Train Your Dragon ("How to Train Your Guardian") was a crossover on its own.
Jack and Hiccup might also see each other as brothers.
Rapunzel comes from the sun, Jack comes from the moon. He's associated with the cold of winter, she's associated with the warmth of spring and summer. Some fans like to think that Jack would visit Rapunzel in her tower, made possible by his ability to ride the wind, and after she left home, they reunited. Rapunzel, however, is the only member of the Big Four, aside from Hiccup, to have a definite canon relationship. Jack and Rapunzel relate to each other on many levels. They both feel a certain level of loneliness (Rapunzel has spent eighteen years locked away in a tower, while Jack has spent three hundred years without anyone to interact with), and a curiosity as to where they come from (Jack wants to know why the moon chose him, while Rapunzel believes the lights are "meant for her" and wants to know why). Their relationship could also work well because they are the only two characters who are immortal in canon (assuming Rapunzel's hair was never cut and she can use her powers to keep herself young forever the way Gothel had).
In fanfiction, the two have a pairing, Jackunzel. This has become quite a popular ship within the Four. This pairing's fanfiction is also the basis of the Frozen Tangled Guardians story where they are the parents of Elsa and Anna.Brave appeared to be set in the wintertime with snow in every outdoor scene, which proves that DunBroch has a somewhat harsh winter. This could be because Jack is a frequent visitor to the Scottish kingdom.
Jack and Toothless have little contact other than through Hiccup. The only thing the dragon and the winter spirit have in common is that both can fly. Given Jack's mischevous and troublesome personality, Jack would take the longest (or at least the hardest) for Toothless to trust completely.
Pitch and Jack are actually very similar. If Jack had made different choices before becoming a Guardian, he could have very well ended up like Pitch. Pitch knows this, and he constantly tries to bargain and blackmail to get Jack on his side.
Pitch sees Jack as one of the biggest threats along with Toothless (him being the most feared and powerful dragon of the known species) and vice versa. He will usually try and use his powers to control the other members of the Big Four in hopes that he can threaten Jack with them or convince him to join Pitch.
There is a belief that Pitch is (in essence) Jack's grandfather. This is because some fans believe that Mother Nature had helped Manny in making Jack into a winter spirit due to her control of the elements. Therefore, that would make Jack Frost Seraphina's son and Pitch Black's grandson.
In some cases, the two are paired up in a romantic relationship.
Being the only human in Pitch's army, Gothel would be able to see Jack through her knowledge and experience with magic. Gothel envies Jack's immortality, as she craves eternal youth for herself. Jack would show some anger to Gothel, from abusing Rapunzel for so long and only see her as a tool for her own purpose. Gothel despises him for not appreciating his immortality, whereas Jack pities the woman, believing she doesn't appreciate her mortality.
Mor'du, being an animal, could see Jack, but has little to do with him. In confrontations with Mor'du, he finds Jack irritating and attacks him ferociously. With thick black fur and a high resistance to pain, Mor'du is more than a match against Jack's ice attacks.
As a dragon, the Red Death is capable of seeing Jack. Given the dragon's humongous size and weight, Jack's powers have little effect, as the dragon's firepower can easily melt ice and snow. Seeing that this is a dragon that destroys anything and everything, Jack and the others use its massive size to their advantage, and seizes the opportunity to freeze the gas within the Red Death's throat before it can ignite, leaving the dragon defenseless and unable to attack.
Friends and AlliesEditJelsa (Jack Frost x Elsa). Fans would suppose they have much in common, like the sacrifice of their lives for their sister (Jack has died for his little sister, while Elsa was isolating herself from her younger sister to protect her), they both wield the power to control snow and ice, their common change hair color (Jack was born with brown hair before getting his powers, while as the producers of Frozen have confirmed that if Elsa was not born with her powers, she would have brown hair like her mother) and also their experience on isolation (Jack has been isolated for more than 300 years, unable to interact with someone while Elsa was isolated in her room, away from the world). The two characters are complementary. Jack did not interact with someone for years just like Elsa, who locked herself alone for years in her room. Each one can understand the other. Elsa is quiet and reserved while Jack is very playful and mischievous. His prankster side could succeed in revealing the true personality of Elsa, which is actually more cheerful and less serious. Writers often theorize that Jack may have saved her with his powers because she was a stillborn as a baby and he felt responsible and watched over her, later falling in love with her as she grew older. Some fanfictions tell that Elsa (after setting an eternal winter of Arendelle) was isolated again in her ice palace, and Jack would remain near her, helping her learning to control her powers. Outside of the romantic relationship, they are also portrayed as having a sibling love or being close friends since they both share the hardship of being isolated for a long time. They balance each other out, Elsa showing him the importance of being serious, and Jack showing her the importance of having fun and "letting it go". This has become quite a popular ship.
Despite her outgoing, child-like demeanor, the odds of her being able to see Jack Frost may vary. Her inability to see him could be a side effect of the events that led to her forgetting about Elsa's powers, either forgetting about whatever adventures he had with them as children or forgetting about magic in general, Jack's connection with Elsa being stronger over the two. In some stories, Anna would be Jack and Rapunzel's youngest daughter, explaining her elder sister Elsa's powers. In other universes, Anna and Jack are paired together in a romantic relationship since their fun-loving personalities fit very well together.
Mavis and Jack, despite being supernatural and immortal, are both very childlike and energetic, and they both love to explore and have fun. The two would very likely get along, probably spending nights flying around the world and visiting different places. Since they have fun-loving personalities, they are often portrayed as being very close friends, and in some cases they are even paired up together romantically in a ship called Javis. Outside of romance, however, the two are still closely associated with each other, since they are two of the few characters in the fandom who are immortal in-canon.
Jack and the Once-ler have much in common, both of them being optimistic, free-spirited boys. Though fans may disagree on whether or not the Once-ler would be able to see Jack, most of them agree that they would be good friends if they did meet. (Since the Once-ler has had experience with the Lorax, some fans assume that he would be childlike enough to see other guardians such as Jack.) "Frostler" is a pairing that the two characters share, many of its fans believing that Jack would be the one to stop (or attempt to stop) Once-ler from making his bad decision of chopping down the Truffula Trees. It is possible that the two might get slightly annoyed with each other at times, but in general they are often portrayed to have lots of fun together, singing, joking, and playing with the forest animals. It is also possible that Jack might be associated with the Lorax, since both of them are labled as "guardians."the Knights of the Epic Brave Dragons, Jack acts as a surrogant "crazy" uncle, instilling fun at the expense of her parent's patience.
Whether or not Moana would be able to see Jack is up for interpretation. Although she is a teenager, she has interacted with demi-gods, monsters and magical beings such as the ocean, so it is possible that she would be able to believe in a frost spirit.
Moana and Jack are both rebellious teenagers with a strong relationship to a certain elements. They would most likely get along well, though she might find him to be annoying at times.
Jack would view Hans as just another humorless royal and wouldn't pay him much mind until he sees how upset the reveal of his and Anna's engagement makes Elsa. In Frozen Tangled Guardians, Jack would be the first to despise Hans, immediately becoming suspicious of the "Prince Charming" that swoops in out of nowhere with a wink and a smile, charming his wife and planning to take his daughter away from him. It would only be more frustrating, considering that he would be unable to tell him off to his face and would only settle on enacting his usual antics on the Prince.
Rise of the GuardiansEdit
When Jack was human he had a younger sister that he cared deeply for. According to the memories, the two often played together, and Jack always tried to have fun with her and the others children of his village. Jack also died because he saved his sister from drowning when she was trapped in thin ice, and that's the reason why the Man in the Moon chose him as a Guardian. When Jack discovered his memories, instead of thinking about his death he was happy when he knew he had a family on his own, as well as saving his sister was the last thing he did. "I had a family! I had a sister! I saved her!" That's where he realized he was supposed to be a Guardian all along.
Jack Frost's MotherEdit
According to his memories, Jack lived with his mother and his sister (his father not being shown in his memories). She was apparently a caring mother toward her children, and Jack helped her taking care of his sister.
AlliesEditRise of the Guardians, North is a Guardian along with Jack, and therefore establishes a strong bond with the winter spirit. Jack and North are friendly towards each other possibly due to the fact they both (on different levels) represent Winter and Christmas. He is often seen as the fatherly figure that is missing in Jack's current life, being the one to persuade Jack into assisting the Guardians despite Jack's reluctance to join them. Out of all the Guardians, North shows the most faith in Jack has a firm belief in Jack's ability to become a Guardian, teaching him about what every Guardian must discover for him or herself, their center, providing an example by explaining his own center, Wonder. He also guides him towards finding his own center even when others seem to lose faith in the spirit. Jack, in turn, has a sense of respect for North, hinting at a desire to break into his workshop to possibly find more about what the Guardian does. North is very protective and supportive of Jack, becoming both a close friend/ally to the spirit as well as a father figure and mentoring force to the wintry trickster.
Outside of their film relationship, North is always shown as being close to Jack, some going as far as to place North as Jack's father or step-father. This may be a result of the fans wanting them to maintain a family type relationship, or simply loving the interactions the two have and how North is always helping Jack out. North is often shown as encouraging Jack's fun nature, although he is quick to step in and help Jack stay on the right path.
Bunny has a complicated relationship with Jack Frost. On the one hand, he seems to oppose fundamentally what Jack stands for. Bunny stands for Hope and new life in spring while Jack stands for fun in the cold lifeless winter. On the other hand they respect each other for some of the same reasons, and it is Jack that helps Bunnymund lighten up and remember why he is a guardian in the first place.
Jack and Bunnymund have a long rivalry with each other that presumably began in '68 when Jack caused a blizzard on Easter Sunday, resulting in Bunnymund holding a grudge against him. Bunnymund was the only one who objected to bringing Jack into the fold of the Guardians and Jack never wasted a chance to irritate or tease Bunnymund, who in turn rubbed salt into Jack's emotional wound of children not believing in him. They were competitive towards each other, seen during the Guardians collecting the teeth (Jack blasting Bunnymund with frost, Bunnymund making Jack fall into one of his holes). However, Jack helped all of the Guardians who, despite protecting children, never spent time with them, to become closer to them, particularly for Bunnymund who grew a soft spot for little Sophie, Jamie's little sister. Bunnymund was touched to learn that Jack helped Jamie continue to believe in him, and they finally became true friends as Bunnymund watched in pride and gratitude when the Guardians officially welcomed Jack into their fold after Pitch's defeat. Outside of canon it's common for fans to depict Bunnymund and Jack as brothers, with Bunnymund being the older of the two.
In the film, Toothiana is a guardian along with Jack, and therefore establishes a strong bond with the winter spirit. She often is shown with a slight infatuation towards Jack, especially with his snow white teeth - but their relationship never expands past friendship and companionship to the romantic level(despite how many fans support the Frostbite pairing between the two). Toothiana is very protective of Jack, and has shown some motherly emotions towards the boy as she discovers his lack of memories and his struggle with his loneliness.
Outside of their film relationship, Tooth is always shown as being close to Jack. This may be a result of the fans wanting them to develop into a relationship, or simply loving the interactions the two have. Jack is often shown as teasing Toothiana, oftentimes because of her petite height. Other authors place her in the same type of position as North, being more of a mother-type figure to Jack.
Jack saves her from being taken prisoner by one of Pitch's nightmares. Afterwards, he instantly gives her the name "Baby Tooth" after seeing her gold feather. Just like Tooth, Jack is protective of Baby Tooth and he doesn't mind her following him around from time to time. In Antarctica, Baby Tooth taught Jack how to use the Tooth Box. As a result, he was able to remember parts of his past and discover why he's a Guardian. It's possible that Baby Tooth has the same fangirl perspective that all the other fairies have towards Jack.
Sandy is a guardian along with Jack, and therefore establishes a strong bond with the winter spirit. He often is shown as a silent companion, although he is largely respected by Jack even before the boy becomes the Guardian of Fun. This can be seen with how Jack already recognizes the Sandman when he sees the dreamsand each night. Sandy appears to really respect Jack, and shows a true belief for the spirit throughout the film.
Outside of their film relationship, Sandy is always shown as being close to Jack. This is possibly because of the relationship Sandy already has with Jack, and how the two seem to have a lot in common that they could relate on. Jack is often shown conversing with Sandy, sometimes as a confidant and other times simply because they both are guardians that keep each other company. It's fairly common for fans to depict Sandy as a grandfather or uncle-like figure.
Jack and Jamie have the closest relationship. Jamie was Jack's first believer and as such is always delighted by Jack and his abilities with snow. Jack usually takes center stage in Jamie's art and craft projects.
At first, the Yetis seem to display some level of disdain towards the winter spirit. This disdain does not seem to extend from any personal grudge like with Bunny, but rather viewing him as an annoying distraction or pest. It is to be assumed that since his induction into the Guardians, the Yetis welcome him into the Workshop with open arms.