Gui Xian (龜仙) in some of the official English versions is a misnomer because "Gui" belongs to a different concept of the Chinese four benevolent animals (Si Ling or Ssu-Ling).
"Origin: China. One of the 4 symbols and guardian of north. It has a turtle body and a snake for a neck. It supports the Earth from below when not exterminating youkai."
Genbu acts as the end boss of Nakano's Stone Site gold-level instance and the last of the four beasts to be implemented in IMAGINE. In his true boss room, players will need to kill waves of boss-sized Mizuchi, Nozuchi and Chimera before killing the illusion form to spawn his true form. After obtaining the plug-in item from his endchest, players will need to speak to the Train Conductor ghost NPC within Ikebukuro's Zoushigiya Cemetary and show him a variety of items to help him remember in order to receive the ability to fuse him. Genbu is a special triple fusion of a variant of Mizuchi, a variant of Tan-Ki and Nozuchi.
Shin Megami Tensei IV
Gui Xian can be found in Midtown. It can teach Flynn the Fang Breaker, Ice Breath, Rakukaja and Bufudyne skills through its Demon Whisper.
Shin Megami Tensei IV Apocalypse
"One of the Ssu-Ling, celestial creatures in Chinese lore. It represents the direction north, the season of winter, and the element of water."
—Shin Megami Tensei IV Apocalypse compendium
Gui Xian can teach Nanashi the Fang Breaker, Bufudyne and Rakukaja skills through its Demon Whisper. It benefits from learning Ice and support skills, however its stats will progress more towards a physical attack build while leveling up.
Devil Summoner: Soul Hackers
"One of the Ssu-Ling, celestial creatures in Chinese lore. It represents the direction north, the season of winter, and the element of water."
—Devil Summoner: Soul Hackers 3DS compendium
Persona 2: Innocent Sin
Unlike the other Personas of the Hermit Arcana, Genbu and Byakko have Great Affinity with everyone.
Persona 3
"One of the Ssu-Ling, celestial creatures in Chinese myth. It represents the direction north, the season of winter, and the element of water."
—Persona 3 compendium
Persona 4 The Animation
In Persona 4 The Animation, Genbu is the Persona that represents the bond between Yu and Eri Minami.
In the True Ending Special, Genbu appears to defend Yu from Margaret's Loki and its attacks, as well as bring a message of support for Yu from Eri.
Persona 5
"One of the Ssu-Ling, celestial creatures of Chinese myth. It represents the direction north, the season of winter, and the element of water. Known to be a great warrior, it supports the Earth from below."
—Persona 5 background
Genbu is the first Persona of the Temperance Arcana and can only be obtained through fusion in the Velvet Room. It is the second of three Personas to learn the Patra skill and the first Persona usable in battle by the protagonist to have access to the Mabufu skill. Genbu is the first of three Personas to learn the Resist Forget skill and the first Persona to learn the Defense Master skill. When itemized through Electric Chair execution, Genbu yields a Patra skill card. Genbu is also used in an advanced fusion with Seiryu, Suzaku and Byakko to summon Kohryu.
DemiKids Light & Dark
"A resurrected beast-demon with unknown powers that have yet to be awakened."
Known as Gembu, he can perform the combo War Breath with Garuda and Hydro Blast with Mermaid. Gembuzom can be created through Relic fusion using the Lion, Slim, Live, the Fist, Dino, Wing or the Fist, Whirl, Slim combos.
Digital Devil Saga: Avatar Tuner
After defeating Ananta in the final floors of the Karma Temple, the Embryon can come down from the tower and find out four powerful demons have taken over several sectors of the Junkyard. Gui Xian appears in Anahata. The Asura Gui Xian is a woman who follows the orders of Huang Long. The battle is quite simple as long as Gui Xian is not hit by Earth spells, either the Embryon's or her own, as they will greatly heal her. Curiously, Genbu's two heads keep on bickering through the battle.
Gui Xian gets three turns per round and the fight is typically played out in three stages. In the initial stage Gui Xian will only use light damaging skills. After more bickering, the second stage will begin and they will instead begin casting Magic Repel every round and switch to mid-tier attacks. After trying to let the party off with a "warning," the party will appear to insist to keep fighting and Gui Xian will not seem too enthused; this begins the final stage. Gui Xian will keep throwing up Magic Repel and switch to heavy damage attacks in addition to using Rage for extra turns.
One problem faced when fighting Gui Xian comes from their extremely high resistance to Physical and Gun attacks, almost nulling them completely. Once the second stage starts and Magic Repel comes into play it becomes difficult to damage them without access to some form of Almighty-based attack. However, if the party casts so much as a single -kaja skill, Gui Xian will start another conversation in which they accuse the party of taking cheap shots. Casting a -kaja skill every round with the character with the weakest magic stat will allow for two magic damage dealers to keep attacking, since Gui Xian will use Dekaja instead of Magic Repel every time.
Shin Megami Tensei: Liberation Dx2
"One of the Ssu Ling, celestial creatures in Chinese lore. The Four beasts were divided equally throughout the land to protect the five emperors in ancient China. There is even a mural depiction of them in the Kitora Tomb in Japan. Gui Xian, the turtle, is protector of the north and represents the season of winter. It gives guidance based on the future it foresee and will even fight against evil in the guise of a human."