I have connected the dots! I think Yvain is Malik Izem, the Sacramento Kings player, for the following reasons:
His name. Malik means King / Owner / Master while Izem means Lion. Who is known for frolicking around on adventures with his pet lion? That's right, Yvain!
He plays in the team "Sacramento Kings". Merlin says that the harbingers are drawn by Arthurian things like how the MC live in Avalon apartments. So it is possible that the same is happening to Malik (unless I don't remember correctly).
Demons are after the harbingers. They even know MC's secret. There's a scene where a TV/monitor is possesed by a demon and it is MC (secret) and Lorelei who appear there, but then it mentions a game between ??? and Cavaliers. It is possible that they mean a match between Sacramento Kings (where Malik plays) and Cleveland Cavaliers (where Eze Ilozumba plays). So it seems that Malik is on the demons radar (and/or Eze).
That's all. I just wanted to share this thought.
And here's another theory for the theorycrafters! I think this is probably the first time that anyone's mentioned Malik or Eze too.