Merlin is a magical, very old, skinny (and boney) wizard. Merlin was first introduced in Shrek the Third.
Shrek the Third
In the 3rd film, Shrek, Donkey, Puss, and Artie come across Merlin, who is revealed by Artie that he was the old high school magic teacher. The school had to fire him because he was insane, he then found a house made out of stones somewhere and started to live there. Merlin helps them run away from Frollo's Horse. Shrek and his team had to be teleported away to Far Far Away but Merlin refused until Artie begged to do it. Since his magic is very weird he teleported them away but changing the bodies of Donkey and Puss. At the end of the film Donkey and Puss asked if they could get back in their own body and that happened only now their tails were swapped.
Merlin has bald hair and he also has a very long beard. Merlin wears a blue and dark yellow wizard uniform with a wizard hat (also with short blue pants, though unseen), white dirty socks, and brown dirty sandals. He also wears glasses.
Merlin is based on the "The Sword in the Stone" character (Merlin). He is voiced by Eric Idle.
Artie: This is lame.
Merlin: But you're lame.