Hardness (金剛) is the sixth episode of the Darwin's Game anime.
- Shuka tells Kaname why she participates in the Darwin's Game.
- In the anime, she learns about the Darwin's Game through her parent's phone, while in the manga she is tricked by a player later on.
- In the anime, Fuli notices Shuka, while the manga doesn't mention it.
- Because Tagonaka Yukimasa doesn't appear in the anime, the diamond ring spawns in the hotel instead and is claimed by Rein and Ichirou.
- Sui claims that the Darwin's Game will give you whatever sigil you wish for.
- Shuka and her efforts to try and track down her parent's killer is covered in the Flag Game novel.