Matthar is an unidentified background character in the first season. He is a newly made Ranger of the Night's Watch, of the same group of recruits as Jon Snow.
Matthar is present during the speech Lord Commander Jeor Mormont gave to the "graduating" recruits. Matthar is assigned to the Rangers.
Following the Breaching of the Wall, Matthar's fate remains unknown.
In the books
In the A Song of Ice and Fire novels, Matthar is also called Matt. Like Grenn and Pyp, he becomes friends with Jon.