Appearances and Incarnations
Lilly's background matters a lot on the story in which she appears. When she is around, her past is usually the same as John's, at least as far as upbringing and life events go. Actually Lilly can be used like an indicator of other story elements, as how she lives can tell us a lot about what John"s living environment is like. Furthermore, Lilly isn't always present, this can tell us a lot as well.
Generally relaxed and chill. She isn't at all unkind, but she is more sensible than John when it comes to helping others. She cares for John in a seemingly casual and sibling-esque way, but her connection to him is shown to matter more to her than she lets on, because when he is put in danger she starts to break down mentally.
Ability and Experience
Lilly usually doesn't have much more going for her other than an above average level of intelligence.