[Ok before I start I wanna say this is my first headcannon so I'm sorry if it's not the best and just my own opinion on how he would be in a relationship]
We all know this man is lonely as heck because everyone walks on eggshells around him so I can see him dating someone who treats him the same as any other demon.
When you guys first start dating it's like a dream come true to him and you. First being him because FINALLY someone loves him for who he is and not his title and second being you because you are gonna be spoiled to the MAX and treated like a QUEEN/KING.
He is such a teddy bear omg we know for a fact that he is a cuddly person so I can see him being really clingy and touchy in a relationship. I can also see him being realllllyyy loyal too get you a man who can do both
I also know that he is busy ya know being future king of devildom isn't easy. But, he would spend all his free time taking care of you and making up for that lost time
As you get deeper in the relationship he starts to open up about his guilt and his past and if he made the right decisions in his life.
I think he's that type to marry you right away because like I said everyone walks on eggshells around him and he believes that you are his soulmate no matter what gender you are.
If you ever got injured he'd treat you like a piece of glass no matter if it was a paper cut or something worse....Now if someone injured you on purpose? Ooh boy that demon would be dead
I can't really see him as possessive of you but will get jealous if the brothers are fighting over you.
You probably won't walk right for a couple days oof
Speaking of not walking right that's the only time he would be rough with you. I can see him being suuuper gentle in bed because he's huge and doesn't want to hurt you you're his queen/king dammit
Your confidence will go up in bed because he is consistent on praising you and telling you how much he loves you
If you're married he'd try to get you pregnant right away if you can that is.
He'd be so good to your kids and spoil them as much as he does to you. But he will not hesitant to punish them if they are misbehaving
Barbatos I don't think would approve of your relationship with Diavolo because he constantly keeps losing focus in important meetings thinking about you but would eventually get over it cuz Barbatos can't keep him away from you lol.
100/10 would date and never look back