Still not fully sure if this is valid but here is some evidence I've found while reading:
Chap. 28.- "What? You traitors. No matter how good looking he is (Mr.Liu), he'll never be as good as our Mr.Yu."- said Billy
Chap. 48.- When Mr.Yu shook Yun's hand and said "Such warm hands" Billy aggressively interrupts.
Chap. 48.- When Yun goes for a drunk kiss on Mr.Yu, Billy Quickly blocks him taking the kiss on his lips instead.
Chap. 49.- Billy admits he's dating someone in the office.
Chap. 53.- Mr.Yu mentions only caring about "the person who's precious to me." And apologizes to Jade.
Chap. 53.- Mr. Yu says "You don't need to worry about this anymore." Shortly before Billy claims the rumors are false.
"My lover said I don't have to hold back! And I can say what I want!"- Billy
"Wait, that wasn't . . ." - Mr.Yu
Chap. 53.- Billy had hated Jade when she first joined the company "And you obviously liked Manager Yu . . ."- Billy
Billy joined the club impulsively saying, "At first I was busy keeping an eye on you two during club activities . . ."- Billy
"You dare admire someone whose taken?! You harpies . . ."- Billy
Chap. 57.- Billy was caught in front of the company building by Jade flustered not long after Jade accidentally eavesdrop on Mr. Yu's argument on the phone, where so and so was worried about Mr.Yu cheating.
"Then why don't you just come here yourself?"- Mr.Yu
"I'm not the one who volunteered you to go all the way out there." (At the time Billy moved to Geojedo to work in a shipyard)
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I have news for you
https://www.mangago.me/read-manga/the_deputy_wants_to_do_it/
Sooooo, is no one gonna mention the secret gay couple? Proud to say I knew since chap 1 lmao. Why didn't we get a confession sene from Billy??? Aaarrrrggggghhhhh!(づ ̄ ³ ̄)づ