Spoiler
From what I understood in the novel, they don’t become orc or whatever monster the helmet is. They keep the human body, but the helmet give them the power by fusing their human self with the spirit of the monster. So their comportment and experience in life will also be influenced. It’s all in the mind realm. Later we learn that there are 2 groups in Eden, one which wear a helmet, and an other which wear a mask. Each group has some differences in objectives. And to further complicate things much later we discover that sometimes, very few don’t wear anything.
During one ark Raon will be kidnapped by Eden or more precisely by Merlin (old woman magician obsessed with him because he remind her of her dear and trusty knight Lockhart) in order to make him part of Eden. He wore the helmet and meet her knight in it. Merlin awaits for Raon mind to be taken over, but the battle happening in Raon mind is won by him. Resulting in Raon absorbing the power and eating the rest of the soul of the Knight.
That’s a spoiler in the lines…all might not be 100% accurate. Hope it helped (๑•ㅂ•)و✧
Am I the only one interested in how that dude transformed into an orc with that orc face helmet??? Σ(°д°ノ)ノ