Kayn... isn't evil. He's an arrogant teenager with a shit ton of skills and would probably have EPD.
But he's not evil. Kayn kills for the purposes of the Order of Shadows, not just for sport.
Rhaast is already a corrupting influence and implied to be a driving force behind Kayn's more sadistic actions.
In this case, I suppose Rhaast's voice lines in default form could be replaced with Zoe lines.
Alternatively, he could have the First Star's antithesis attached to him, or alternatively some minion of the Darkness.
To be honest, he could work really well as the Blue guardian of a team that's trying to shake off the influence of Darkness.
A team like...
Kayn, seeking more power, he accidently invoked an ancient being of Darkness and now has the literal personification of darkness attached to him, will he fight it off going full Star Guardian, or fall to it, letting Rhaast manifest.
Kai'Sa, when Kayn awoke Rhaast, she was able to return to the normal world, having been lost to the Darkness for years, she strives to fight back by passing the knowledge she's gained onto new generations of Guardians, starting with helping Kayn overcome Rhaast.
Katarina, though scarred while fighting the Darkness alongside her sister Cassieopeia, she still stands strong and resilient, looking for some way to fully purify her sister.
Cassiopeia, she's been mutated by Darkness, her lower legs warp into a twisted tendril of Darkness, as she tries desperately to escape her gem.