I'd say it's Sissel, mainly because they look the cutest as a couple while Philip and Owen obviously have something going on between if you chose Sissel's route.
Owen and phillip have a thing in sissel's route because the mc isnt there to intervene. But if you choose owen's route he actually confesses that he has a crush on the mc, literally doesn't make sense because he flirts with everything he sees XD
The other two routes are much shorter, so have less time for anything to develop between the other characters, but Sissel has that hero worship of the Black Cat, and Owen and Sissel were childhood friends, both of which could be developed into something romantic.
Of course they dont have a canon love intrest but if there was one, even though I'm playing Owen's route and i like it a lot, i believe Sissel would be his canon love interest
