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.