1904: J.M. Barrie’s circle of young friends included Margaret Henley, daughter of the poet W.E. Henley. She called Barrie her “friendy” – or rather, her “wendy”, since she couldn’t properly pronounce “friendy”. That appealed to Barrie, who used Wendy as the name for a leading character in his play Peter Pan – its first use as a girl’s name.
Source: Lisa Chaney, Hide-and-Seek with Angels: A Life of J.M. Barrie (2005), p. 217