i believeit is erin's mom, one the mom may gotten skills from previously being in the scouts, and witnessed the deaths and the horrors of being in it, thus shetells her son not to leave her. she might have left the scouts for such reasons and changed her identity, like hair color name, which could eplain why the titan's hair is blonde. also she could also have knowledge and access to her husbands experiment and chemicals which granted one the ability to turn into a titan, and may have been her husband's first subject before prexperimenting on her son. and one last thing, she comes for erin, her long lost son since the incident, she wants him back,since she never wanted him to leave her, like all mothers, and sought to find him,and teach him and love him. also, after her son was taken from her a second time, from her mouth in titan form, she left there sad,sonless again, and she begins to cry. no matter what, especially not a trained spy , will anyone cry for eren likethat, besides miksaand armind. but they aren't titans so that leaves his mom. people also say it cant be his mom becus her spine was broken and she was eaten, well erin was also eaten, and what happened, he rejuvenated, which is what his mom could have alslo done. after being rejuvenated she could have sprung out of the titan that eat her like erin has done. also having been an adult when injeced with the titan sirum , andknowing, unlike erin, and having 5 years, she had time to master all of its abillities like focusing on the growth of a certain limb faster or crystalizing. she may also be able to change into more form than just the female one like the colosal and the armored.
another idea, as said before,but also be a spy, who's against everyone, and the scouts and king, and wishes to see his destruction, coiuld be why she lays waist to the land and its people and why she tells erin also not to join the scouts because she doesn;t want to kill him amongst the others that she will. the father may also been in the esponiage with the mom, or against her but cant stop her so he huries to protect his son with the sirum to protect him from his own mother and hher destruction.which is why he escapes to avoid any danger. if they were together he may be the armored or the colosal titan.if these theories are wrong consider an addition to those.his fathers against his mom.she's aspy along with him and a gropu of others. his fathers leaves the group becuase he has guilt from helping so much people to have them die again, so the m,other along with her group steal the sirum and uses it on themselves, so to avoid having the destruction be tied to himself he leaves, but not before equipping his son with the power to stop them, since the dad may be a coward or just can't bring himself to stopping them