Hels is used to forcing masking on himself due to the nature of the people in his home realm. This included trying to put on a tough persona after countless people who found him out would trigger his RSD on purpose. He'd try to avoid hyperfixating on things, he had to suppress stims, and it got painful.
Once he began his battles with Wels, he had no idea that the knight could read him like a book. He only realized Wels could see past his mask once his statements during battle became more and more obvious attempts to get him to drop his "act". One day, he chose to just smile and drop his sword, telling Hels that he didn't need to fight anyone to prove he wasn't weak, and that his experiences made him strong already.
From that point on, Wels helped him break his habit of masking so he could just be himself, and it made him happy seeing Hels becoming comfortable talking about interests and stimming and just letting himself relax—he eventually got Hels to stop worrying about "stepping over boundaries" whenever he felt like hugging him.
One of the moments when Wels felt proudest of Hels was when he felt comfortable enough to admit he'd gotten hyperfixated on him, admitting his feelings. Wels knew what Hels wanted to do, so he told him, "You know you don't have to hold back with me worrying about boundaries." It was enough for Hels' mind to give him permission to kiss Wels, who happily kissed him back.
Hels pretended at first to hate Wels for displaying the traits he repressed. Now that he feels safer, though, he can show his admiration.
