"One of the four oni Chikata Fujiwara was said to have controlled during the Heian period of Japan. He could cause floods with the swing of his arms."
—Shin Megami Tensei III: Nocturne compendium
Sui-Ki is one of the four Oni that were part of the Mantra demons' elite force. But they made a mistake which angered Thor and they were forever banished from Ikebukuro. The once elite Oni group became mere petty thieves who mugged anyone going through their scattered hideouts around the Ikebukuro Tunnel.
Sui-Ki's hideout is located around a trapped area within the tunnel after going through a pitfall. He presents himself as a cold demon who would leave those who demonstrate courage alone. If the Demi-fiend feigns cowardice, Sui-Ki will attack.
Upon defeat, Sui-Ki expresses curiosity towards the Demi-fiend in his last few breaths, openly wondering if he's a demon or a human.
Shin Megami Tensei IMAGINE
Sui-Ki can only be obtained through fusion. He appears as a field boss on Shinjuku field when summoned by one of the charms the Yaksini NPC in Ichigaya Camp's B11 will trade for Magnetite.
Shin Megami Tensei: Strange Journey
"One of the four oni controlled by Fujiwara no Chikata, it can cause floods at any time. In the Taiheiki, Ki no Tomoo succeeded in expelling the oni by reading a Japanese poem."
—Shin Megami Tensei: Strange Journey compendium
Shin Megami Tensei IV
Sui-ki has placed a Domain over a relic on Shinjuku's East Entrance Main Street. Defeating him obtains the Corked Bottle relic needed for the optional main quest Bring Back Wine.
Sui-Ki can teach Nanashi the Bufudyne, Rakukaja and Mabufudyne skills through his Demon Whisper. He benefits from learning Ice, Physical and support skills.
"One of the four oni controlled by Fujiwara no Chikata, it can cause floods with the swing of its arms. In the Taiheiki, Ki no Tomoo expelled the oni by reading a Japanese poem."
—Persona 5 background
Sui-Ki is the fourth Persona of the Moon Arcana and can be found as a Shadow in Kaneshiro's Palace and the Kaitul area of Mementos, with the title “Floodbringer Demon” in the vanilla version, and "Raging Water Demon" in Royal. He is one of three Personas to learn Headbutt and the first Persona usable in battle by the protagonist to learn the Bufula skill. He is the only source of the Null Nuke skill and the first Persona to learn Mabufula. When itemized using Electric Chair execution in the Velvet Room, Sui-Ki yields a Mabufu skill card.
Sui-Ki first appears in Kaneshiro's Palace as a Shadow that acts as an enforcer for Shadow Kaneshiro. They will first appear alongside two Oni during the Phantom Thieves' initial escape from the area. They will later be encountered again as one of the guards in the Banker Passageway, holding a key that the party needs to proceed deeper into the palace.