Is your character a magic user/otherwise magical in some way?: Yep.
If yes, can Artemis "sense" this?: Absolutely.
If yes, is there any specific impression that he would get from them?: Definite affinity for ice, but he can use various types, not all of which are elemental. So mostly just magical in general, as death gods are supernatural beings. And yes, he'd come off as powerful. With the ice element more than anything else.
Anything else?: Yay for not looking his real age?