Honestly, out all of the minor character inhabiting this series, Buttercup is easily my favorite.
I don't know what makes her so appealing before my eyes, but I guess that it has a lot to do with the fact that they gave her some fitting lines -compassionate but also inquisitive about their presence there- and that her voice-acting was one of the most accurate on the series (a far cry from Blackavar and other not-so-well chosen rabbits).
There's also the fact that her design is JUST above generic and that really doesn't drive attention from anything else to focus on her. Some may call that lazy, but I think it is perfect. She's a one-shot character and there's no reason for the overkill, with an over-the-top kind of design.
I would have liked to see more about her, to be honest.