Diane Thurston is Axel's granddaughter and Renton's older sister. She and Axel were the ones who raised Renton after Adroc's death. She left home some years ago, and for initially unexplained reasons her family has not heard from her since. Despite appearing in Axel's family photos and in a silhouette in the series opening, her face isn't shown for the first 35 episodes —under many different circumstances, her face in flashbacks and photographs is always covered. Much of the first few episode's narration are made as if Renton was writing letters to his sister.

Talho is troubled by Holland's past relationship with her, and reacts with anxiety at her mention. Diane was obsessed with her father's research; so much that it affected her relationship with Holland. The terms of their breakup weren't thoroughly explained, but it's linked with her desire to more deeply understand Adroc's work.

At some point, Diane somehow ended up in the command center of the Scub Coral. There, in a library artificially created by the Scub Coral, she and a small group of people read to understand the truth they seek. The vast knowledge they wish to seek would take almost a lifetime and all that remain are under their own volition. Even though Diane told Renton they'll stay together, the Scub Coral command center was blown away and Renton had to leave in order to save Eureka and the kids. It's suggested that the command center will return again and that Adroc and Diane are alive and well, continuing their reading.