spoilers
he looks like that because in the volume 7 finale art's body is so messed up sylvie sacrifices herself to fix him.(his core is shattered still) which is why he looks like that, since he is half asuran he can see aether particles without the help of realmheart he eats aether and fixes his core which turns it into an aether core. tessia is the "vessel" which is cecillia, during a rescue mission tessia fights nico and is captured. agrona reincarnated cecillia into tessias body, tessia is not elijahs lover, cecillia is