- Devil Children Black Book & Red Book: Protagonist (Black), Major Character (Red)
- Devil Children White Book: Minor Character
- DemiKids Light Version / Dark Version
- Devil Children Fire/Ice Book
- Devil Children Mobile
Devil Children Black Book
Setsuna Kai is an orphan living with his brother Nagahisa Kai. One day, a mysterious parcel is delivered to their apartment shortly before a group of Petit Frosts arrives to kidnap Nagahisa under Lucifer's orders. Opening the package, Setsuna finds a Devil Riser and a Vinecom, as well as a horn-shaped object. Using the Devil Riser, Setsuna summons his companion Cool in order to defend himself, but he can't stop the Petit Frosts from kidnapping his brother. Setsuna runs outside and meets a mysterious boy named Zett Takajō, who tells Setsuna that the horn-shaped object is one of the Messiah Horns and uses it to send Setsuna to the Expanse.
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 the real Lucifer is his father. After defeating Azel and rescuing Lucifer, 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 battles and 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 Book
Setsuna lives in the same apartment building as the previous game, next to his half-sister Mirai. Masaki Kuzuha first meets them inside of a cave in the Ice Expanse, where the two tell Masaki that they were suddenly attacked by the demons sent after him due to also being Devil Children, and are thus investigating the issue.
After King Frost is trapped by angels and rescued by Masaki, Setsuna and Mirai tell Masaki that the best course of action should be going directly to Tenkai so they can try to settle things peacefully with the angels. Masaki is chosen for this job since he's friends with Takaharu Ougi, an Angel Child.
After Masaki tries and fails, he falls into the Deep Hole. There, Masaki is rescued by Setsuna who tells him to go to the Center Expanse and see Lucifer. At the end of the game, Setsuna, Mirai, and Lucifer are seen in front of the door leading to Altimia. They are unable to get in as only angels or Angel Children can open the door, leaving them no choice but to ask Masaki for help.
After beating the game, Setsuna can be found in his apartment. He can then swap partners with Masaki.
DemiKids Light & Dark
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 has the same voice actor as Yosuke Hanamura.
- Under normal circumstances, Setsuna speaks as informally as one would expect a 10 year old boy to do, but in White Book, when speaking to his father in court, Setsuna starts being humble and polite. He even goes as far as changing his personal pronoun from the rough-ish ore to the more demure boku.
- In White Book, Setsuna and Mirai are unable to open the door to Altimia as they do not possess angel powers. However, in both Black Book and Red Book, Nagahisa gives his powers to the protagonist after being defeated. This means that one of them should have been able to open the door.
|Playable||Masaki Kuzuha - Clay|
|Non-Playable||Takaharu Ougi - Rei - Shou Ougi - Paku - Haamiru - Setsuna Kai - Mirai Kaname - Cool - Veil - Takajo Zett - Nathanael - Raguel - Lucifer|
|Expanse||Earth Expanse - Wind Expanse - Ice Expanse - Deep Hole - Center Expanse - Grass Field - Crystal Ring - Cosmos City - Altimia|
|Terms||Devil Riser - King Riser - Macca - Magnetite - Expanse - Devil Child - Angel Child - Devil Database - Devil Phone - Vinecom - Power Unit|
|Lists||Items - Demons - Skills|