Mr. Callahan is a character in Fallout 4.
Mr. Callahan was a resident of Vault 111. He died from asphyxiation inside his pod like most of the other vault dwellers.
The Overseer's terminal mentions a malfunction in Pod C3 where the occupant had heart palpitations and nearly thawed out, and refers to the occupant as "her" even though Mr. Callahan is the dweller assigned to this pod.
Mr. Callahan appears only in Fallout 4.