First making an appearance in Episode 7: Into the Jungle, the three kittens are introduced while being carried by a large Pterodactyl until rescued by Kutal, Mushra, and Sago. Rei and Sen present themselves as twins who "have been together since their lives began and will be together to the very end." Estie adds to it that "she is not a twin, but just younger" and agreed that she was everyone's favorite. Now reunited with Kutal, they find themselves wanting to stay with their family.
However, it was considered 'dangerous' for them to tag along and therefore in an attempt to scare the kittens into going back home, Mushra, Sago, and Kutal sent them on a "Test of Courage" through the jungle. The three walk through a graveyard, over a river on a log, and up the hill to the goal. Estie finds a human doll in the river as she accidentally falls in and is pulled out by her brothers, which she gives to Yakumo and pushes them forward on their journey to find the remainder of the humans.
Afterwards, the kittens, now having proven themselves not to be scaredy cats, travel with the four to the school, where they wander off and explore, discovering the triceretops that lingers inside the abandoned building. Once finding their way back to the others, Estie stays with Mushra and Yakumo while the twins go with Kutal to the kitchen. When Yakumo needs a moment as she is swept into nostalgia, Estie forces Mushra out of the room with her strength and claws. The Twins are berated when acting out of line with the kitchen appliances and are told by Kutal, "You were not raised by dogs!" Upon danger striking, all three kittens are told to run and only emerge when the building is demonlished and seeing Kutal and Sago in their Hyper Forms.
The kittens are often displayed with Hakuba, in times when the three Enterrans are fighting, such as when Yakumo is turned to stone and they assist Hakuba in fishing her out of an acidic lake she had been thrown in. Estie strays from her brothers often, going to be look out as Mushrambo fights and is often found cheering for the warrior.
In Arc 2, they are displayed watching the battle with Lanancuras from afar and rebuilding the world with the rest of humaity. At the end of the series, they are shown joined with Kutal, Sago, and Binka, asking which direction to go. Estie replies with, "To the West!" As Yakumo always said. The series alludes to that they travel alongside the others in search of Mushra.