Mr. Tumnus is a peace-and-quiet loving faun who lives in a cozy cave north of the Lamp post. He meets Lucy Pevensie in the Western Woods one day, befriends her and takes her back to his cave for tea for the sole purpose of lulling her to sleep with his flute and then turning her over to the White Witch. But he is too kind to do so, and helps her get back to the Lamp post so she can return to her own world. Lucy gave him her handkerchief.
For this act of bravery and selflessness, he is arrested by the WW's secret police (he manages to give Lucy's handkerchief to Mr. Beaver before his arrest) and is taken to her castle, where he is later turned into stone.
He is revived by Aslan some time later, and he and Lucy have a joyful reunion. When the Pevensies are crowned the Kings and Queens of Narnia, Tumnus is the first-named friend to be rewarded and honored by them. He accompanies a group of Narnian nobles including Susan, Edmund and Lucy to Calormen, and it is his idea to fool Rabadash by stocking the Splendour Hyaline as if for a party, and escape back to Narnia.
Later, it is he who informs the Narnian royals that the White Stag has been sighted. He is seen again at the Great Reunion in Aslan's Country.