Smilodon
Friendship and the Cosmos.
It's mind-boggling to think that Sanjo and Hasegawa were thinking that far ahead. When was Kazu introduced? Was he introduced after Isaka's death? It'd be plausible for Sanjo and Hasegawa to have written Double Forever's story beforehand and implement that into the series story, but wow. Some careful planning there, contrary to how OOO's story worked behind the scenes.