Karin
For the first few hours of Covenant Karin was actually my favorite character and I nearly made a site about her. So suffice to say, I like her a lot and find some of the fandom's hatred for her pretty surprising. I've seen her viewed as a threat to the Yuri/Alice romance, which confuses me. As we learn at the end of Covenant, Karin is Yuri's mother. This should put her out of the running as a romantic interest, if nothing else.
Anyway. Karin and Yuri start off very different from the way Alice and Yuri do. They immediately develop a rapport that is definitely based more on the similarities between them than the differences. Perhaps because of the aforementioned spoiler, they share plenty of personality quirks. Not that things are sunshine and rainbows; they get into arguments and bicker. Eventually they support each other; Yuri in the Neam ruins and Karin when it comes time to tame Amon.
I've always been of the opinion that Karin was Yuri's best friend. There's no doubt in my mind that they loved each other but they had different kinds of love. Yuri wanted her as his best friend and confidant while Karin's love was definitely of the romantic variety. But this difference doesn't stop her from still being his best friend. She never seriously made the moves on him -- she supported him and stood by his side. I think Yuri needed that more than anything at the time. He was without a purpose and he needed to surround himself with people who would help him succeed in finding one again.
For his part, Yuri was aware of the potential between them but never acted on it. The last thing he wanted or needed was another girlfriend. He wasn't done mourning Alice until the middle of the second disk and never fell out of love with her. Though Karin filled her role in a lot of ways -- as the person to talk him out of a rut and get him back on track by simply believing in him -- he never conflated the two women or used Karin as a substitute. Karin was Karin and Alice was Alice. They were both very important people in his life.
