I have a friend who is very optimistic, cheerful and friendly, but also quite childish and impulsive, and sometimes downright crazy (she once spray-painted her sister purple because she stole her chocolate). She is very likeable for it, perhaps you could make the optimistic fairy like that?
Maybe she could occasionally get distracted from the problem at hand, and generally not take things too seriously. For example, the optimistic fairy might be halfway through her answer to a problem when she realises you are standing in front of a cake shop, and instantly goes off on a tangent "I think we should let the p-SHINY! WANT! GET NAO!!!!11!1!!111!!!!eleven!!!". Having random outbursts like this every so often, on some of the more minor problems, could build character. She should probably be restrained on the larger, more important problems though.