Wow. I don't know why, but somehow this manga really got to me.
My interpretation is that Kishi made a deal with someone to stay in the human world during elementary school and if someone falls in love with her during that time, she can stay there.
Yuu's love for her and them being together all the time is what gave her enough strength to stay way past elementary school.
But when she started distancing herself from Yuu, she became too weak to stay in the human world and had to go back to wherever she came from.
And when she turns 18, I guess her "powers" will have matured enough to allow her to go to the human world by herself and stay indefinitely.
I'm not sure what kind of being Kishi is, but honestly I don't think it really matters here.
Presumably you could read this as a metaphor, to an extent, where Kishi is a lesbian and thus different from the other girls. Yuu recognizes something in her, possibly a sense of shame at being how she is, and her acceptance of Kishi makes that darkness recede. Yuu's seeming rejection makes her fall back into her old self, although she's gotten good at faking it around others. At the end, Kishi is happy despite needing to leave, which itself may be a metaphor for not taking things further for now, but will be back, or free to pursue their relationship, after they're grown and free of the constraints of high school and beyond the "it's just a phase" years. In this scenario, the blood drinking and healing would represent intimacy, with Yuu letting Kishi past all barriers, although not necessarily sexual.
Of course, none of this is likely the case, just me musing "aloud". Pay it no mind.