Downplayed: Instead of a whirlwind girlfriend who sweeps Bob off his feet, Alice comes into his life as a new friend who helps him think outside the box, and doesn't fix all of his life's problems, but gives him a bit of perspective — and he can return the favor, because Alice has a life outside of Bob.
Bob actually has Hidden Depths that Alice manages to detect and bring out.
Alice has a zany personality because she's a child.
Alice is a manic pixie dream girl, who often helps Bob out, but she does get her own story line and development so no one can say her character exists solely for Bob.
Exploited: Unknown to Alice, Bob is actually a thief. When he and Alice go around town, he provokes her into public stunts which distract his marks while he robs them.
Defied: Soon after meeting, Bob and Alice end up despising each other; Bob considers Alice a loud, unpredictable pest, and Alice considers Bob unbearably dull. When somebody suggests they should date, it's hard to tell who is more disgusted.
Discussed: Charlie is of two minds about a girl like this. His Uncle Bob tells him how he met Alice and she turned his life upside down in a good (or bad) way.
Alice brings a lot of excitement into Bob's life. Unfortunately, this is because she is seriously mentally disturbed: a yandere, a Stepford Smiler, or otherwise in need of seriouspsychological help. Bob is portrayed as either the poor bastard whose life goes to hell because of her, or a bastard who is taking advantage of her.
Alice tries to make Bob's life more interesting, but instead, he gradually wears her down until she becomes just as ordinary as he is.
Alice and Bob start dating, but her wild personality proves too much for his Straight Man personality to handle and they break up.
Bob becomes quite emotionally dependant on Alice, to the point he can't do anything without her. And when/if she leaves...
Alice feels that she has to make Bob's life better, but Bob is just fine with his life and considers Alice an annoyance.
Alice purposefully seeks out guys who have problems, brings them out of their slumps, and makes them totally dependent on her in a sick All Take and No Give relationship.
Alice sees Bob as her latest "project" and dates him to "make him better" and gratify her hero complex.
Alice tries to liven Bob's life up, but only ends up ruining it, causing him to hate her.
Reconstructed: Alice and Bob eventually develop as people, and reach an agreeable compromise; Bob breaks out of his shell and becomes more willing to try new things out, while Alice settles down and becomes a mature (but still fun-loving) woman.
Played For Drama: Alice acts like there's no tomorrow and does random crazy things on a whim... because she has 6 months to live and is trying to fulfill her bucket list.