Pitch is the Red Death's rider. They have taken to each other as both have extremely vicious natures. Pitch, on occasion, fuses his nightmare sand with the Red Death's firepower to create powerful weapon and extremely strong blasts.
Mother Gothel and the Red Death have little interaction and not much in common with the exception that they both wish to destroy those who had defeated them in the past.
Mor'du and the Red Death appear to be friendly rivals, if one can call these two friendly. They both wish to be the dominant warrior of the group and frequently compete in "friendly" duels.
Hiccup and the Red Death are mortal enemies. It was Hiccup (along with Toothless) who defeated the Red Death at the end of How to Train Your Dragon. As such, the Red Death will be extremely keen to get even with Hiccup. The Red Death was also reponsible for the loss of Hiccup's left leg. Many like to compare the battle between the two to the story of David and Goliath.
Originally, Toothless was in the service of the Red Death (retrieving food for it.) When Toothless met Hiccup, however, the two quickly bonded and Toothless showed Hiccup the Red Death. Hiccup and his friends then helped to defeat the Red Death, though at first it seemed like they were about to lose the battle.
Being an animal, the Red Death is capable of seeing a Guardian like Jack. Jack's powers of ice have little effect on the dragon's exterior due to its thick hide and its breath can melt Jack's ice from a great distance. However, if Jack could ever manage to freeze the gas within the Red Death's throat before it ignites, then it would be completely unable to attack.
Though she can neither ride a dragon nor cast bolts of ice, Merida still possesses a minor threat to the Red Death. Her abilities with a bow and arrow mean that a carefully aimed shot into the Red Death's eye could blind it. But since it has six eyes and normally relies on hearing and smell, it would hinder it less and only make it more angry.
Rapunzel has a gift with animals (her chameleon Pascal is a prime example of this.) She has the ability to calm and empathise with animals of any kind. This however would have no effect on the Red Death; it's mind is far too savage and aggressive to pay attention. Because she poses no immediate threat however, the Red Death is unlikely to take any notice of Rapunzel, which gives her a stealth advantage.
Friends and AlliesEdit
Hans could potentially admire the destructive strength and powerful control the Red Death could exert over the other dragons, but would most likely prefer not to use it as a weapon, for Hans prefers to earn the loyalty of his subjects than subdue them.
Because of Helheim's Gate's barren, semi-lifeless environment, Mandrake would find comfort in the Red Death's domain. As for the Red Death, the sheer size of the dragon would leave the two nearly unnoticeable to each other.
Despite the Red Death's size and force, Elsa could be one of the few characters in the fandom capable of holding their own against the Red Death without any help. Since her powers do not seem to posses any limits aside from their uncontrollable behaviour, Elsa could direct the entirety of winter itself onto the dragon and freeze it solid in its tracks. However, because the dragon's brute strength and its ability to breath a gust of flames is capable of destroying an armada of ships, it could possibly combat Elsa's powers enough to pose a threat.
It would be hard to describe what would happen between Princess Anna and the Red Death. One theory is that once she got over her initial fear, she would be able to help fight the monstrous dragon. This theory draws off the fact that she: a) pulled down an entire tree b) flung a stone bust across the room like it was nothing c) punched Prince Hans of the Southern Isles off the side of a boat. This can lead to the conclusion that she is pretty strong.