Hardness (金剛) is the sixth episode of the Darwin's Game anime.
- Shuka tells Kaname why she participates in Darwin's Game.
- In the anime, she learns about Darwin's Game through her father's phone, while in the manga she is tricked by Miho later on.
- This scene in turn makes Shuka's father a Darwin's Game player; whether he had this status in the manga is unknown.
- In the anime, Fuli notices Shuka, while the manga doesn't mention it.
- Because Tagonaka Yukimasa's manga scenes up to this point in the anime were cut, the diamond ring spawns in the hotel instead and is claimed by Rein and Ichirou.
- Sui claims that Darwin's Game will give you whatever sigil you wish for.
- Shuka and her efforts to try and track down her parents' killer is covered in the Flag Game novel.
- Shuka's father used her birthday (March 26) as his phone's PIN.