It's the eyebrows - and the snark, his dialogue is so funny. And the way he loves Merlin. And the tragic past, and the collaboration with killing magicians, and the going out to bravely die for Merlin when he's not a natural hero.

I totally cried when the voice-over was happening, of the letter he sent in the first season. Like OMG NOOO GAIUS CAN'T DIE!