I think there is a large number of explanations for why Sinara followed Kasius, apart from her saving him on the battlefield back when (which is really something to establish faith the opposite way). But there's only one that has got to be true: She's a female Kree, and we have never seen another. She's a female Kree warrior, and we have never seen another. If Kasius was already in the dumps, after desertion in battle, what's breaking with tradition and taking a female Kree warrior as his guard and confidante?

That thing he did in the end, smearing his brother's blood on his face and taking her hands and "All our dreams have come to bear!" I'm just like, What Is Your Relationship?

It's true that we've never seen any other female Kree, and I definitely see it as a caste/status thing. She seems to defy all the Kree conventions, and the fact that she was referred to as The Stray seems to also be a marker of how low-born or whatever she originally was. Maybe she was like Yondu, sold as a slave? But what she seems to really like is how Kasius values her intelligence as much as her battle skills- "I need you to think! Where will she go?"

Yeah, I like how Mack and Yoyo splitting off from the group is consistent characterization instead of giving us audience members what feels most comfortable, lol. They promised each other they wouldn't lose themselves in this dystopia, and they're holding to it. That's all sorts of awesome!