S. Coups
He would definitely get upset and yell quite a bit, but as long as they weren't doing it to get information, he wouldn't kill them immediately.
"That guy? Are you kidding me? Have you gone blind?!"
Jeonghan
He'd be happy that you found love in such a horrible place, but be very worried that they were using you.
"Well... as long as you're happy. I'll always be here for you."
Joshua
Of course, as a spy he would have all the detail to know that if the rival you've fallen in love with was trying to use you, so he wouldn't worry.
"Oh, him? Yeah he's alright, besides the fact that he tried to off Mingyu once. But, yeah, he's wonderful!"
Jun
He'd be the type of brother who would be extra protective over his little sister, so it's no surprise that he would always show up when you had plans with the rival
"Have we met before? Ohhh, that's right! Remember that time we stole half of your drug clients?"
Hoshi
When you told him, he would be ignoring everything you said until you mentioned the rival. After that, he'd do anything to keep you from seeing him.
"Sorry [Y/N], you're on lockdown for a few days. I heard word that some gang was looking all over for you!"
Wonwoo
He wouldn't care much, but his torture supplies would always be on the ready incase you were backstabbed.
"It's none of my business who you associate with, just don't be a dumbass."
Woozi
He'd find it cute that you were in a relationship, however he would always be warning you of danger or making sure that you were extra cautious.
"Oh, that's nice! I never thought he was meant to be in the mafia anyway. Though... people do always have hidden sides."
The8
Having mixed feelings about it, he wouldn't know what to do. In the end he would probably end up just leaving it alone unless something happened.
"You realize this could be a big risk, right? Guys like him eat little punks like you!"
Mingyu
He would actually seem happy for you, but he truly saw it as a sneaky way to easily overthrow the rival's gang.
"Wow, I can't believe you actually got through to them! That takes a lot of skill."
Dokyeom
Like how he deals with all of his problems, he fixed it with another problem. You could only see the rival if you beat him at Blackjack.
"I'm feeling particularly nice today, maybe I'll let you do best two out of three!"
Seungkwan
He definitely wouldn't like you being with a rival so he would be very crabby anytime you were going to leave the house.
"Oh, are you going to see that boy? I would bet you a million dollars you won't be coming home ever again. Wait, you won't be able to pay it cause you'll be dead!"
Vernon
Not liking it one bit, Vernon would go out of his way to make sure you always had things to do so you wouldn't be able to see the rival.
"I forgot to tell you- Seungcheol wanted you to spend the day with him so you could learn the basics of how to protect yourself!"
Dino
He would be like Hoshi, but worse. He would straight-up tell you that you wasn't going to let you leave until you had forgotten your feelings.
"As the best brother in all of the world, there's no way I'm going to allow you to be with that asshole!"