Mother is the boss of stage 9, this is where the monsters spawn from and is like the heart of West Mansion, this is probably the most difficult boss to defeat, as it keeps spawning monsters during the fight to defend itself. Once Rick kills Mother, the West Mansion is set on fire.
In Phase Nine: The Ruined Heart, Rick reenters the West Mansion to destroy the heart. Once Rick destroys the heart, he's send back in time and back outside the mansion. The Mother boss in the remake is neither a boss nor an enemy.