Diane Thurston is Axel's granddaughter and Renton's older sister. She and Axel raised Renton together after their father Adroc died. She left home years ago and went silent—no contact with family since. Despite showing up in old family photos and a silhouette during the series opening, her face stays hidden for the first 35 episodes. Whenever she appears in flashbacks or pictures, something always obscures it. Early on, much of Renton's narration comes across as letters written to her.
She was deeply obsessed with understanding their father's research, to the point where it wrecked her relationship with Holland. Talho still gets visibly anxious whenever Diane's name comes up, clearly bothered by whatever went down between her and Holland back then. The breakup itself isn't explained in detail, but it boils down to Diane's fixation on diving deeper into Adroc's work.
Eventually Diane ended up inside the command center of the Scub Coral itself. She joined a small group there in an artificial library created by the Coral—a place where they could read and study to uncover the truths they were after. The knowledge they're seeking would take lifetimes to fully grasp, but they're choosing to stay and keep going. Even though Diane promised Renton they'd be together again, the command center got destroyed when Renton had to leave to save Eureka and the kids. The ending hints that the command center will return someday, with both Adroc and Diane alive and still reading.
Content compiled by AnimeList.moe from publicly available sources.
