Baek Heesong is Baek Yugeon’s (our MC’s) older brother. The older brother was hospitalized (in a coma) and the commander took advantage of our MC’s inability to pay the bills to keep him at his side. If Yugeon (our MC) finds out that his older brother disappeared, he will do his best to escape because there would be nothing to tie him down anymore (other than the commander’s will).
And commander hates that the MC slept with Taein. He's experiencing jealousy for the first time.
Even though that is true, I don’t think that’s why he was trembling. I think the commander was feeling real terror (intense fear) that Yugeon was going to leave him if he found out his brother was missing.
So who's baek heesong ? And why he trembling like dat?