First off Eremes isn't a player character. He was an MVP in RO1 so if you're so intent on keeping the same as RO1 then why the-_- can we talk and chill with Eremes instead of killing him? Why are cards so -_-ty compared to RO1's cards. Why are the graphics so different. Why this, why that. Do you realize how stupid you sound?
This is the game you're playing. Get used to it. Cause its not gonna revert back to RO1.
The Eremes we fight if I remember correctly were failed experiments in the Bio Labs. They were creations of what were to be "super soldier" replicas of the top warriors Midgard has ever seen. This appears to be the one they tried to replicate and has either returned from an assignment outside the kingdom or has for the first time shown his face in several hundred years. (Given the life span of demi humans has not actually been discussed in the game's time line at all.)
And as for the transformation argument... The human body is limited in every single way. That would be like Bruce saying going Hulk makes him weaker just because we see the Hulk and not Bruce himself. As an assassin we are limited by our physical body and as such to show we've surpassed limitation we meld with the shadow beast inside us, and take it's power for our own. So we become a new being that does not suffer our feeble limitations as humans. That is being "in control" of the beast within. Appearances aren't everything, it's all about what is the raging war inside. In this case you mutate but your heart, mind, and soul are fully in control of your new body. If you aren't running around just randomly punching stuff, you're in control.