hello! hope you're enjoying your tama!
mechanically, there's no difference between the characters besides how healthy they are. well cared for adult characters like mametchi and ginjirotchi will live longer, while nyorotchi and tarakotchi (bad care characters) will not. Of course, each character is said to have a personality that isn't represented in the device itself; for example, mametchi is a genius and a science lover.
the hearts probably represent the filling of the happiness bar. just decorative, pretty much!
