ymir fritz
Any detail about this character is a spoiler. Don't read them until you read the 122nd chapter of the manga or watched the 21st episode of the fourth season of the anime!!
She was the slave of the founder of the Fritz dynasty and the first one to acquire the power of the titans. She was forced to bear his children as a "reward" for her servitude. After her death, he forced her three daughters to eat her corpse to inherit her titan powers, which split further into the nine titans.