What defense can be used when one assaults someone to protect a loved one? I have heard it called self-defense but I’m not sure that’s right.

In this particular case the attacker says he blacked out and cannot remember what happened. When the police arrived he, his pregnant lover, and the victim were the only ones in the house. His lover was unconscious after an alleged attack by the victim, the attacker entered the house, claims he saw the victim standing over his lover and cannot remember anything until the police arrived. Could you use a self-defense plea, or possibly a mental disease or defect plea? What defense would be more substantial and can you quote a case were this plea was used and successful?
Also, the lover cannot remember anything beyond arriving at the house. She does not recall being attacked, although there are injuries. The police found that the victim in this case was the father of the baby and think that is possibly motive for the attacker. The attacker claims he had no knowledge on this fact until after he was arrested.