Setsuna Kai is a character from the Devil Children series. He is the protagonist of Devil Children: Black Book and his partner is the Cool the Cerberus. He's known as Serina in DemiKids.



Devil Children Black BookEdit

Setsuna Kai is an orphan living with his brother Nagahisa Kai. One day, his brother is kidnapped by Petit Frosts under Lucifer's orders and Setsuna ends up summoning his companion Cool in order to defend himself. After defeating them, Setsuna inteds to rescue his brother and is taken to the Expanse by Zett Takajō thanks to the Rainbow Messiah Horn.

Setsuna wakes up in Central Land, where he's soon found out by Lucifer's guards and taken to Hel's prison in Ice Land. After escaping and finding out that he's a Devil Child, Setsuna learns that Nagahisa was rescued by angels and so he keeps defeating the Tyrants of the Expanse in order to obtained access to other Lands and find Nagahisa. In Sand Land, Setsuna learns from his mother's ghost and Isis that Nagahisa is actually Setsuna's half-brother and an Angel Child, the reason for his kidnapping being to prevent the angels from finding Nagahisa and abusing his power.

After hearing from Zett that Nagahisa went back to Earth, Setsuna heads there and finally reunites with his brother. Nagahisa tells Setsuna that the Messiah Horns and the Messiah Eyes are to be used in the Orgel Room to either activate Armageddon or Ragnarok, two programs that will decide the fate of the world. The angels want to use the former to reset the world as they see it as sinful, while the demons want to use the latter to advance the world towards a new era. Nagahisa tells Setsuna that he'll be siding with the angels, and that as a Angel Child, he's bound to be Setsuna's enemy, as he is a Devil Child.

In Forest Land, Setsuna learns from Mammon that the Lucifer who's currently ruling the Expanse is actually Azel pretending to be him, and that Lucifer is his father. After defeating Azel, Setsuna confronts Nagahisa and his half-brother's father Michael in the Sanctuary. Nagahisa tells Setsuna that they shouldn't have to fight and asks for his cooperation. If Setsuna says yes, it leads to the game's first bad ending. Ragnarok is activated and the game's credits roll. After that, Setsuna is left alone in a world of nothing. Mirai appears and the Devil Children wonder what they should do. Mirai notices that the world is restarting and that this time they should make things right. This will automatically start a new game.

If Setsuna refuses, he and his father Lucifer fight against Nagahisa and Michael. After being defeated, Nagahisa tells Setsuna that their fight is over and he can go ahead and activate Ragnarok. Setsuna feels Nagahisa's power coursing through him, remarking that he has the power of humans, demons and angels inside of him as one. Setsuna heads inside the Orgel room and summons Hoshigami, who asks Setsuna if he wants to create a new world or become a god.

Choosing the latter leads to the game's second bad ending. Setsuna defeats Hoshigami, becoming a god and ascending to a higher plane of existance. On Earth, Mirai asks Nagahisa about Setsuna, who tells her that he doesn't know who she's talking about. Mirai then remarks that Setsuna disappeared from the world. Choosing the first option activates Ragnarok, marking the start of a new era for both Earth and the Expanse, restoring both.

Devil Children White BookEdit

Setsuna lives in the same apartment as his half sister Mirai Kaname and the two are dragged into Masaki Kuzuha's quest when demons attack them by mistake. He can swap demon partners with Masaki in the epilogue.

DemiKids Light & DarkEdit

Serina is a S-Rank BattleNet opponent. Cool and decisive in battle; he desires to be the best in the BattleNet. After giving the protagonist the second demon, he will tap into his latent demonic power as an inverted star appears on his forehead, with his hair becoming spiky and pointed. He gives a Nex after 15 defeats, Meta Nex after 25, Neo Nex after 30, and King Nex after 40.


Setsuna Kai by EO Artist Yuji Himukai
Setsuna as he appears in Devil Children Mobile
Setsuna Anime Design
Setsuna as he appears in the anime
Setsuna Black Red Art
Setsuna as he appears in Black Book & Red Book
GBC Setsuna
Setsuna as he appears in Black Book