I'm still pretty new to CoC2, but I would have to say that Kiyoko takes first place pretty handily in my opinion. She's got easily the most fleshed out relationship with the PC in the entire game, both her sex scenes and non-sexual interactions are all absolutely incredible and varied, and the fact that she actually has flaws and very clearly struggles with raising Kinu without your help really sets her apart from nearly every other character. After playing TiTS to completion numerous times and finally deciding to experiment with another one of Fenoxo's projects, I really didn't expect to see my fiercely intelligent and competent fox-wife reprimanding our daughter and potentially inhibiting Kinu's early development in order to hide her own sense of inferiority in a game like this, and it immediately drew me into her and Kinu's story. There was clearly a lot of time and effort put into the kitsune family and it shows in every aspect of your interactions with her.
Second place would definitely have to go to Kinu herself because of how much time you spend raising her and making decisions that affect how her personality will turn out in the future. She's kinda at an inherent disadvantage compared to other characters due to the fact that you obviously can't fuck your own daughter (I mean, some people might want to, but there's rules against that as long as Fen is funded by Patreon), meaning that she misses out on a lot of content that other characters, like her equally complex mother, have access to. Despite this, the amount of time you spend getting to know her and helping raise her to the best of your ability still manage to put her way higher on my list of favorites than most other characters in the game. I really appreciate the fact that there is no "right" way to raise her and I actually did struggle with some of the decisions I made as a result. Much like Kiyoko, I didn't expect this level of complexity out of a game as unfinished as CoC2 and I'm so very glad I was wrong.
And finally, third place has to go to Kassyra. While she's still unfinished and certainly more basic than Kiyoko and Kinu, I greatly enjoyed her fleeting appearances throughout the story so far. She's a legitimately interesting and intimidating villain who has her own backstory, and I very much appreciate that. She provides a good power ceiling to the universe by demonstrating her vast magical power and ability to instantly reshape your body to her whims, and both her servants throughout the world and regular appearances by the demon herself continually reinforce that sense of power. After playing TiTS for so long and having to deal with the fact that there really isn't a true villain of the game aside from PROBABLY Maximilian Steele (who gets mentioned offhand a grand total of ONCE), it was really refreshing to encounter an opponent who demanded respect and caution for once. Even beyond that though, there's just a lot to like about her in general. I like her over the top design, I like that her backstory actually has a decent level of complexity to it and is doled out in a couple pieces at a time instead of just dropped on us in order to make her sympathetic, I like that you don't even get the opportunity to fight her so that she can maintain that sense of intimidation, I like the fact that you have the choice to submit or resist her demands to fuck you, and I absolutely love the way she toys with and talks down to you at every opportunity while also clearly taking an abnormal level of interest in you that makes you feel like you're just playing into her hands regardless of what actions you take. Her content is extremely limited by the nature of only having a handful of one-off meetings with her and her story arc isn't even close to complete, but she hooked my on the game from the very first encounter and only grew more interesting from there, so I'd say she's more than worthy of a mention.