Unless you want to explore polyamory, and I am poly myself so I would love that, Asterion, Storm, and Pedro already have their romance needs fulfilled. I think it would be wasteful to focus on another romantic story for any of these three, would distract from well needed exploration of the other characters. Unless you threw an extra character in to make a throuple.
Out of Luke, Kota, Themba, Khenbish, Robert, and Argos...
Robert and Themba - They both have a similar vibe, kinda quiet and intellectual but in different ways, with Robert being more metaphysical and dealing with souls and the afterlife and Themba dealing with finances and assets. Also Robert and Thembas answers in the drinking game match them up fairly well. Might be a way to explore both characters a little more and they're in need of it. Robert might also provide opportunity to explore the mythology behind Themba and his family.
Luke and Kota? - I can see it being a lot of people's answer, given just how many times Kota brushes Luke off about the sexual stuff it's hard to tell for me. These two have the most time to work with if choices are incorporated into whether or not romance blossoms between characters.
I still don't feel like I have scoped out what kind of character Khenbish is compared to the others honestly, but anyone who loves to eat or is into wrestling is a good match for him, it's maybe that simple. Kota and Luke are probably the foodiest characters.
Nikos and Asterion by far have the most potential but they're also uh...cousins. And I can't see anyone else other than main character for Nikos?