I noticed this after I heard an Alter say "For the King!" after capturing.

King - Unknown
Queen - Elise (Spider Queen)
Knight - Hecarim (Horse)
Bishop - Karthus (Wears the right clothing)
Rook - Mordekaiser (Only one that is weird, although his armor is kind of... castley.)
Pawn - Evelynn (She's an assassin; therefore a pawn. She's the one that sets off "For the King")

Draven.

Mordekaiser being the rook isn't that weird. His heavy armor makes him the closest thing to a castle.

And Yorick?

I feel Morde would be the King, Maokai the rook.

You forgot about Yorick.

Being an avid Chess player, this is all I will think about today. Thank you. brb school

Yorick is the only one that doesnt fit.

What