Skullbreaker is an unlockable character within Call of Duty: WWIINazi Zombies, he can be unlocked by completing a full challenge set in the map The Shadowed Throne, in this case being the "The Answer" challenge set.
Challenges
Crush Them Quick. - Responded to a bad situation swiftly and effectively ("Go Go Go!")
Get to Wave 21 in under 25 minutes.
Time Cruncher - Dismantled a machine of death and terror before nightfall. (Hard shutdown.)