Navi is a fairy who accompanies link on his adventures, providing help along the way. She only appeared in Ocarina of Time, where she also provided info on enemies when the user wished. In The Legend of Zelda: The Wind Waker, she is replaced to an extent with a magical stone which allows Tetra and her crew to communicate with Link from elsewhere. Midna takes her place in The Legend of Zelda: Twilight Princess.
Many fans criticize Navi's help system, as it often abruptly pauses the game for Navi to provide her info, which many players find annoying. Navi will often provide info on even the most obvious details, furthering her annoyance. Much of her info, however, is optional. Also criticized is Navi's tendency to say "Hey!" every five minutes until Link listens to her, which many players find obnoxious.