When Kisara is stabbed with Demon Kanna’s spear and she touches it, she suddenly gets a rush of her memories, which include a young Shuu. Kisara tells Shuu to flee at once, Sharon grabs him and grabs hold of the runner of Ayano’s chopper to take them away.
Kisara charges at Kanna, but at the last minute is stopped dead by another memory of Kanna as an innocent child. In that instant of hesitation, Kanna strikes Kisara down and she falls into the sea. Kanna soon follows her down there when Mikhail fires the satellite beam at her twice.
Kanna is dormant on the sea floor, but could reawaken at any time. Meanwhile Kisara is in hospital and won’t wake up or heal at her usual speed. All Shuu and Ayano can do is sit there, wait, and contemplate what comes next. Sharon makes clear that as far as her bosses are concerned Kanna is an S-Class Demon that must be destroyed.
The problem is, none of the contractors in Bayron City are sure they can deal with an S-Class even with a united front, and instead place their hopes in Kisara, who they don’t know is in a bad way. While alone with Kisara that night, Shuu makes a heartfelt plea to her for what he should do, and she wakes up and kisses him.
Unlke previous kisses, this one seems to transfer Shuu’s memories back to him. Starting with his sudden breakup with Ayano and resignation from AAA, to teaming up with/seducing Sharon, to finding Kisara, whom we learn is a distant blood relative of his, thus making their contract possible.
Forming a more efficient and practical contact with Kisara involves a lot of trial-and-error, along with an actual paper contract that’s several hundred pages long. Before they make things official, Kisara reads the whole thing through and, unbeknownst to Shuu, makes a couple of changes.
For one, she makes a kiss the means by which the limits of her demonic power are unleashed. This wasn’t how the contract was initially written up, but the kissing gesture was inspired by how Shuu “formed contracts” (i.e., bedded) previous humans like Ayano and Sharon. And once she kisses him, there’s no going back.
That brings us to the other thing she changed: if they kiss while their hands are intertwined just so, their contract will be terminated. That’s what she seems to do in their present-day kiss in the hospital, and unless I’m totally misjudging things, this results in all of Shuu’s memories returning to him.
This also means all the memories leave Kisara (they were moved without being copied), so when their lips part and Shuu asks her what the hell she just did, her first words are “Who are you?” Kisara believes Shuu has fought enough and wants him to leave the island and live the rest of his life in peace.
Breaking their contract is how she believes that happens. How she’ll deal with Kanna without a contract remains to be seen. But if Shuu indeed has all his memories back, that means all the drive and motivation to carry out his original mission must have returned as well. In any case, I highly doubt he’s about to abandon Kisara, Ayano, and Bayron City.