The deep intimacy and trustthey have in one another. Zed had so little trust in anyone and met Kayn, who was willing to trust him nearly blindly. I like how Kayn deeply admires Zed, how Zed was the only person in his life that hadn't given up on him. Them finally finding, in each other, something that had been denied from them their entire lives.

I like how subtle their would be, how they would express their intimacy through vulnerability. Zed probably wouldn't verbally say "I love you" to Kayn, but he would show Kayn where he keeps his hidden blades, and allow Kayn to know his body's weak-points, the equivalent of a cat showing its belly. Or how Kayn would show his love by putting Zed before everything, even himself, even willing to set aside his ambitions for Zed.

If Zed wanted to, he could manipulate Kayn and truly break him; If Kayn wanted to, he could kill Zed in his sleep. But they won't do that, because they are each other's everything.