Well according to previous chapters, Arhad was quite literally the devil that was obsessed with a goddess (past Ianna) that was the first to show him genuine kindness. It was never gonna be a normal relationship to begin with because before regressing to the current timeline Arhad has been wanting to have her even before they were enemies
I don't think there will be any change in their relationship, at least not in the foreseeable future. They don't love each other. Arhad/The Devil is a greedy, obsessive being and his number one obsession is Ianna/Roberstein.
They weren't enemies in the previous timeline. She was his enemy, sure, but all he was trying to do was literally beat her into submission. It didn't work; she would rather die for her pride.
So, this time, he tries his best to honor her and respect her pride. Ianna, on the other hand, just likes how obsessive he is with her sword. Her sword is her pride, so after being beaten before, she finally decides that if he wants her that badly, he can have her--but only if they reincarnated.
Her reasoning might also have something to do with Roberstein, but that hasn't been divulged yet. All we know is after Arhad gets everything he wants, including Ianna, the Devil's heart, the world, etc., he is going to take her to Pandemonium.
Sorry, I mean I **believe** they don't love each other. Others seem to mistake Arhad's obsession and Ianna's willingness to serve him as love. Sure, they're affectionate, but I don't really see romantic love here, maybe just platonic.
Maybe that changes at some point? If they kiss, I'll change my mind. ╮( ̄▽ ̄)╭

Much more if they heard what those two are talking like seriously, Mental collaborator? depend of him? Doesnt want anybody to take interest on her? I just facepalm myself how many chapters do they have to realize what kind of relationship they have