I really 愛 Kellan! He was, as much of あなた have said, the Emmett I draw in my head while reading! But, erm, related to your question, I know there's no one as Kellan, but I think that also Stanislav Ianevski could've make a good Emmett. I think it would be sort of funny, 'cause Stan and Rob ,they acted together in Harry Potter & The Goblet of 火災, 火 and they are really best friends. It would've been kind of cool.
Please! Don't misunderstand me! I 愛 Kellan as Emmett! He's the PERFECT one!