Quick answer
Wendy is a girl name meaning "friend".
Meaning and history
In the case of the character from J. M. Barrie's play Peter Pan (1904), it was created from the nickname fwendy "friend", given to the author by a young friend. However, the name was used prior to the play (rarely), in which case it could be related to the Welsh name Gwendolen and other names beginning with the element gwen meaning "white, blessed". The name only became common after Barrie's play ran.
Notes and context
Usage: english