He Raised Her To Be Good

Unicron found a femmeling after a battle with his brother Primus. Not able to make himself kill her he actually feels bad for her and takes her in as his own sparkling raising her with the help of one of the original Primes.

Prima had longed to raise her own sparkling so she goes against her order to kill unicron when she has the chance and instead helps him. The sparkling named Matriarch ends up on earth once she is grown not knowing it's the planet her sire forms the core of. She ends up staying with the autobots but remains factionless.