SPOILERS
(This is about the anime not the manga)
Okay so in ep 11 s1 Yu freaks out and goes all crazy right? Like he demons out and stuff (Idk what the proper name is sorry). So when he does that he keeps repeating (Kill all sinners, all sinners must die ect) So when he tries to kill Shinoa and Mika stands in-front of her and Yu stabs him instead, Yu keeps pushing his Katana (I'm guessing that's what it is as it doesn't explicitly state it but it looks really similar so I'm going with that) through Mika to get to Shinoa, instead of killing Mika. Essentially saying how Mika isn't a sinner in Yu's eyes, even when he isn't exactly himself. Also when Shinoa moves to hug (?) Yu to snap him out of whatever, Yu moves to attack her instead of Mika. The fact that Yu didn't kill Mika and that Mika isn't a sinner could strengthen that Mika is the other Seraph. But we kind of already know that because in an earlier episode Krul (I think) says "Now one of my Seraphs is bleeding out and another one has escaped", the one 'bleeding out' being Mika.
This might not even be true but it makes a lot of sense to me. (Also first time posting in this fandom woo!)
