beta
I think Siwoo liked Byul first and there's something in the past that happened that made him supress that feeling (maybe because they are both alpha, or maybe it is tied to why Byul stopped his idol training). But I think majority of the reason why he wants to suppress is insecurity/anxiety.
And I'll bet when they start a relationship, he will be super clingy to Byul, the exact opposite of what he is now