First of all, it's not a love triangle instead its a square. And who is going to end with our mc, we don't know. In my opinion, it either Sunghyeon (the black hair guy in the cover) and the other is Soohyun (idk why he is not present in the cover) . They both have 50-50 chance (again in my opinion). And the last but the least Jaemin (the blonde hair guy in the cover) I won't say much about him but he also like our mc(Daon) I think so.
Well him not showing in the cover is I guess what the title Secret relationship implies to, at the start they weren't showing his face only his back when daon still didn't confess to jeamin and soohyun was threatenig daon to tell jeamin about their secret affair and that they're meeting behind jeamin's back
before i start this manhwa (or not-) tell me which one is the endgame from the cover