We can only wonder as to who that arm belongs to. It could be Rin, it could be Kaito.

Looks too manly to be Rin, I say Kaito

Kaito :8
Rin cannot have that huge arms :]
Kaito + Len!! x3

how many times has he been told to not go to church.

Maybe it's Meiko. The difference in their size could be causing the arm to look mannish. And in a lot of fan work she is depicted with a (very) subtle masculine appearance.