Mr. Tumnus is a faun from the Chronicles of Narnia continuum. A kindly little man with goat legs, he befriends Lucy Pevensie during her first trip into the Wardrobe, and becomes a close friend to her and her siblings.
Mr. Tumnus lives in a cave in the Lantern Wastes when Lucy comes to Narnia in The Lion, the Witch and the Wardrobe. Fearing the rule of the White Witch, and knowing her hatred of Daughters of Eve and Sons of Adam, Mr. Tumnus tries to put Lucy to sleep with a flute so he can turn her in. At the last minute, however, he loses the heart to go through with his plan, instead warning Lucy of Jadis and sending her back through the wardrobe.
Upon Lucy's return with her siblings, she discovers Tumnus' cave ransacked, with a note on the door declaring him to be under arrest. They don't discover until later that Edmund had met Jadis before and accidentally told her of Tumnus aiding Lucy. Tumnus is turned to stone in Jadis' castle, but is later restored by Aslan along with all the rest of the Witch's petrified victims.
While the Pevensies rule as Kings and Queens of Narnia, Mr. Tumnus serves as an advisor to the siblings. He is present with Edmund and Susan when they meet danger in Tashbaan in The Horse and His Boy. Tumnus dies at some point after the Pevensies leave Narnia, but reunites with them in Aslan's Country in The Final Battle.
Mr. Tumnus is often shipped with Lucy, due to some badficcers' inability to see their close friendship as anything other than a romance. He is sometimes paired with other non-human characters, usually as crack pairings.
Tumnus' meek personality is frequently turned up (down?) to ridiculous levels, making him a servile minion to, or even fearful of, the Pevensie children. While he does behave this way around the White Witch, his relationship with Narnia's true queens and kings is much closer and more respectful.