Keith Shadis is the head instructor of the 104th Trainee Squad. He seems to be a friend or have some sort of relationship with Dr. Grisha, as he knows that Eren is the son of the doctor and praises Eren for his effort.
He is a former Commander of the Survey Corps, and was in command when Wall Maria fell in 845.
Manga: Ch. 15 "One By One"
Anime: Ep. 1 "To You Two Thousand Years Later"