This is what Cassis told Roxana after he "kidnaps" her from the Agriches.
So it happens during the Agriche Downfall arc. Roxana fainted due to blood loss and excess poison flowing through her blood while the Agriche mansion was surrounded by flames. She used the abilities of the poison butterflies too much, and it affected her body a lot. She was walking away from Agriche because she didn't want to stay there anymore.
According to Cassis, Roxana's expression looked like a person who was "looking for a place to die". And that really shocked Cassis, so he became even more determined to take Roxana with him and take care of her in Pedelian.
Roxana, in response, told Cassis that such things were normal for her. As the master of the poison butterflies, her body would not be able to last any longer, and once she dies, her butterflies will feed on her flesh until nothing is left of her. Cassis then kneeled in front of Roxana and told her that if she is going to die anyway, then she should give the remaining time of her life to him. He wanted her to be by his side until her life ends. （/TДT)/
Roxana said that she is fine with any place as long as it's not Agriche, so she more or less agreed to live with Cassis in Pedelian. Cassis holds her hand and kisses it, as if to honor his oath. Roxana said that Cassis's eyes had a dark glow to it when he did that (Cassis borderline Yandere/Possessive moment?) (▰˘◡˘▰)
I will continue with pt. 2 in a separate comment.
I SWEAR ON MY GAY ASS THAT ALL THESE YURI/SHOUJO AI SERIES ARE JUST ABSOLUTE FVCKING *CHEF'S KISS* MASTERPIECES!! HIGH QUALITY ART AND STORY IS ENSURED SO READ AWAY AND ENJOY, MY FELLOW GAYS
This is Part 1 of my ongoing list btw!! There’s a continuation of this list uwu please do follow that as well for more quality gayness if you want~
I found this comment on YouTube by '@Haddy The Bored'. An interesting thing they said was that (in Anime) when Hanako gave the mermaid scale to Yashiro, he said it'll bind them in this life and the next life! Possible bittersweet, reincarnation ending? I went to go check the manga after this, in the 1st chapter they both swallowed the fish scale and their "fates have been tied together." "...They will be placed under a dreadful curse, but tied together by an extremely strong bond." Could it be their curse was to experience a tragic ending together, but meet again in their next life?