Hiro and Tadashi are shape-shifting aliens who are sent to planet Earth to study the human race but Tadashi had to do must of the work because Hiro is irresponsible with his shape-shifting and he's thinks humans are 3 things... smelly, gross and ugly plus he doesn't like to be in human form
Aunt Cass, the human who’s boarding them without knowing about the alien thing, is convinced that the brothers are keeping an odd menagerie of pets, because every time she goes to the upstairs apartment they’re renting, they seem to have a new animal lounging around on the couch. It has not yet occurred to her that she’s never seen said animals and Hiro in the same room, because she’s already convinced that the boys are really weird. It doesn’t help that even Tadashi, the more “expert” on human culture, still hasn’t quite grasped all the concepts of human behaviour. But even though she thinks they’re both really weird, Aunt Cass also thinks they’re both very sweet, and has a soft spot for them, which is why she often forgives the strange noises coming from upstairs and the crazy pets.
The boys are quite fond of her in turn, even Hiro who thinks most humans are gross. They grow so fond of her, in fact, that when their home planet starts discussing using the Hamadas’ research to try to conquer the planet, they immediately argue against it because they want to protect her. Cass might not be the same species as them, but she’s family to them now, dangit, and they’re going to keep her safe even if they have to fight off their whole planet!
- Baymod