So far, we have five confirmed titan shifters. While four of them come from outside the walls, Eren is a shifter who was born inside the walls and is a curiosity. Because of his similarities to his mother and the fact he is definitely her biological child, people tend to say that Grisha’s injection caused Eren’s titan powers.
Grisha was at least aware, as he did speak about that ‘power,’ but all we know is that the injection caused memory loss— possibly the loss of memory that Ymir and Bert were talking about when Reiner knocked him out, that memory being who he ate to gain his power.
This is where popular theories split in two. One theory says that Eren ate Grisha himself, the other saying that Grisha is still alive and is Ape Titan. However, the latter is something that others consider ‘impossible,’ citing the fact that he didn’t know what the 3DMG was. And while there’s multiple explanations for this, one that I’ve come to the conclusion of is that if he was a titan shifter, he must have given himself the injection as well. And if he has given himself the injection, it must have caused memory loss, possibly more severe than Eren’s, where he cannot remember such things.
Regardless, you can’t deny the similarities between the two. If you were to erase the monkey characteristics in their nose and mouth, they two look similar. The eyes, the part of the hair, and general face shape when ignoring the nose and protruding mouth, make them look more than just slightly alike. Basically, they look as similar as Bert and the Colossal, or Reiner and the Armored. It’s a similarity that you can’t exactly throw out the window, especially when Grisha had, for some strange reason, went missing like he did.