|First Appearance||Devil Children Black Book|
|Demon Partner||Cool the Cerberus|
|Japanese VA||Shōtarō Morikubo|
- Devil Children Black Book & Red Book: Protagonist (Black), Major Character (Red)
- Devil Children White Book
- DemiKids Light Version / Dark Version
- Devil Children Fire/Ice Book
- Devil Children Mobile
- Shin Megami Tensei: DeviChil: Protagonist
Devil Children Black Book
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.
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 keeps defeating the Tyrants of the Expanse in order to eventually reach Central Land and take on Lucifer. 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, though the angels did rescue him from Lucifer's Palace in the end.
Setsuna eventually meets Mammon, who tells him 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, who want activate Armageddon and purge the world in cleansing flames while his father Lucifer wants to activate Ragnarok to bring peace to both the human world and the Expanse.
Should Setsuna decide to side with Nagahisa, Armageddon will start and the world will end. The game's staff roll will then play followed by a New Game being automatically started. If Setsuna decides to side with Lucifer, the two of them will kill Michael and Nagahisa before Setsuna confronts Hoshigami. Setsuna is then given the choice to start Ragnarok or battle Hoshigami and become a god. Should Setsuna choose the latter, he will defeat Hoshigami and remake the world. In this new world, Nagahisa is alive but Mirai Kaname is the only one who remembers Setsuna's existence since he ascended to a higher plane of existence by becoming a god. If Setsuna chooses the first option, both the Earth and the Expanse will recover from the damage inflicted by Azel's rule.
Devil Children White Book
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 & 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.
|Playable||Mirai Kaname - Setsuna Kai - Cool - Veil|
|Non-Playable||Nagahisa Kai - Elegy - Zett Takajō - Lucifer - Azel - Hoshigami - Zebul - Michael - Diana - Dust - Paimon - Isis - Souta - Hel|
|Expanse||Ice Land - Marble Land - Sand Land - Forest Land - Fire Land - Central Land - Sanctuary - Deep Hole|
|Other||Vinecenter - Shop - Bank - Hotel - Casino - Training School - Suspicious House|
|Songs||Go-Round - LOVE SICK - Tenkai no Aurora ~ AURORA HEALING - Idol ~IDOLL~|
|Terms||Devil Riser - King Riser - Macca - Magnetite - Expanse - Devil Child - Angel Child - Devil Database - Devil Phone - Keys of Norn - Vinecom|
|Lists||Items (Red/Black - PS) - Demons (Red/Black - PS) - Skills (Red/Black - PS)|