By complex do you mean “most developed”? I guess it would have to be the 3 other Pets during the vampire route, Vicilla, Lauren, and Edith. You learn a fair bit about their personal lives, though you arguably don’t spend as much total game time with them as you do with Amber, in the main route.
Still, all of them are leagues behind Bianca, who’s been in the game there since it’s initial release, as an optional enemy, that can be killed and forgotten about no less!