User blog comment:Gourgeist/Who Is The Beast Titan?/@comment-184.185.152.35-20141007165454

I also have asked the same question, I have a theory:

1.) Grisha Yeager: Seeing that he has the formula that can fuse titan blood into humans and make titan shifters. Also, the "Beast titan" is the only one that talks, not like thinking, but actual talk to Mike.