Manic pixie dream girl (trope)
From Book Trigger Warnings
The "manic pixie dream girl trope" occurs when a book contains a quirky and eccentric character (usually a female, sometimes magical) whose purpose is to convince the protagonist (usually male) to embrace/explore life. This character is often presented as a love interest and has little or no character development.
For a list of books on BTW with this trope, click here.