beta
This is just my hot take, but we already know Byul is a dominant alpha as stated from the previous story.
However, people need to realize that "not all alphas are academically or physically gifted". It's ridiculous to assume that alphas have to be "handsome, smart, athletic, etc."
But back to Byul already being established as a DOMINANT ALPHA, I'm sure Siwoo is either a beta or he's a recessive omega who hasn't manifested yet.