I think the problem with Kiriyan's Bill might be more of an issue of casting than performance. Kiriyan just wasn't quite the right fit for the character.

Bill is so simpleminded. He's twitchy and flaily and rather childish. So much of the cuteness and romance in the story hinges Bill's adorably stupid, hyperactive, puppy-like personality. Kiriyan has an aura of seriousness and complexity to her that I think prevents her from being a fully convincing airhead.