
They are both fat middle aged gay men living together in a big ass mansion with lots of dogs
Takara: HONEY! COME DOWN STAIRS! NOW!
Isaki: Why honey I am feeding the dogs right now THAT YOU WERE SUPPOSED TO DO!
Takara: COME DOWN STAIRS BEFORE I DRAG YOUR GNARLY ASS HERE
Isaki: Ok Ok what is the matter, dear?
Takara: WHAT IS THE MATTER? EXPLAIN THIS? YOU MANWHORE!!
*It was a cake box from the cake shop across the street*
Isaki: Uh. Um. Honey, I can explain. This is not mine I swear
Takara: *crying*
Isaki: Honey no.... I promise it was a one time thing. For the last time. i promise I won't go to another cake shop ever again. Forgive me, I would never do that to you.
*gives Takara enough kisses to get him to reciprocate*
*they have crazy sex* -the end

Maybe I missed something or I totally forgot but when and why did they suddenly join the story? Can anyone explain?

Hello, translator here. So far, the reason the boss bailed hasn't been explicitly stated yet. However, these kind of stuff (the boss or members running away with the money, murder, etc.) happens a lot in the kinds of job Sungho, Joseph, and Yongsu were in.
I think it was ch8? Where the author showed how much debt the characters had, and I think it's related to the boss or the job. So I think what happened was, the boss ran away with their money (the one they were supposed to receive, but for some reason hadn't; yet) and they're still in debt (Not sure with Yongsu as he wasn't in that debt according to ch8. Maybe he has some other reasons he needed that money) so Yongsu got pissed and decided to take revenge.
Maybe we'll see what really happened sooner or later.
Urgggggg