In the Big Hero 6 manga, Hiro was treated differently and bullied because of his brains, similar to the way Elsa was because of her powers, so both may share an understanding. Also, both know what it's like to lose a sibling. Most of Hiro and Elsa's relationship would center around their understanding of each other.
Some fans chose to make their relationship a love hate relationship at first and then make them fall for each other as they begin to realize their similarities. Others make Hiro feel sympathetic towards Elsa and he tries to help her.