If the throuple route doesn't come to fruition then it has to be Kotoko. She's the one who's responsible for how much Takuya has grown and changed by his own admission. And he's already fully ingratiated himself with her siblings to the extent that they welcome him as if he's family. She fell first, and arguably hardest, and her affections toward Takuya are rooted solely in who he is as a person. That isn't to say that Kei's feelings are shallow, but Kotoko wears her heart on her sleeve in a way that I just can't imagine it getting broken.
I'd be sad to see Kei lose. But I'd be fucking devastated to see Kotoko not be with the guy who's obviously perfect for her. She wants marriage and kids and he's cut from exactly the right cloth to make her happy.
...Also I like kurogyaru.