User blog comment:Llama919/Possible Beast Titan Identity?/@comment-6019258-20150903015402

As many have said here, I just don't think the two look that much alike to be the same person. This coupled with the fact that his dad lived inside the walls (and hence would know about the 3DM gear) makes it really unlikely MT is Erwin's dad. More likely there's some relationship to Grisha. They look kind of alike, both wear the same glasses, and have the pointy ear thing in their titan form. I think Ymir, Grisha, and MT (pointy ears) are/were part of some faction that opposes the RBA/homeland forces (bodies that are almost skinless). Right now they've formed a temporary aliance to achieve some shared, important goal.