Sihwan is Sia and Baryu's childhood friend. He is a human who knows about the vampires. He is Baryu's companion and also serves as his 'emergency food' as Baryu calls it.
He also cares deeply about Baryu. He took care of him when he was in a state of depression after being rejected by Sia.
When Sia and Baryu fought, Sihwan served as the bond between the two, guiding Sia towards making the right decisions.