Lugh Lamfada or Lugh the Long Hand, the name Long Hand came from his long spear, Brionach. The great sun god of the Celts, also called Ildhana. Grandson of Balor and killed him in the great war between the Tuatha D'anaan and the Formorians. He fathered Cu Chulainn with Dectera, daughter of the Ulster's chief druid, Cathbad.
"A sun god of Irish lore. His name means "flashing light." He is skilled in many arts, carries his spear Areadbhar, and is known as the Long Arm. He is father to Cu Chulainn and is said to have many wives, including Bui. His grandfather Balor was also his greatest foe; during the battle of Magh Tuireadh he pierced Balor's evil eye with a magic stone."
—Soul Hackers 3DS Compendium
Lugh is necessary in order to fuse Zeed into the hero Siegfried.
Megami Ibunroku Persona
Known as Ildanach can only be fused by using the Totem Dordona's Oar with the spell cards from Cath Palug and Cromm Cruach.
DemiKids Light & Dark
"Karfu's lord and master. An excellent fighter and a master of the spear."
Known as Roog, he can perform the combo Razor Dance with Karfu and has the power Magnify.
Devil Survivor 2
"A sun god of Irish lore. His name means "flashing light." He is skilled in many arts, carries his spear Areadbhar, and is known as the Long Arm. He is father to Cu Chulainn and is said to have many wives, including Bui. His grandfather Balor was also his greatest foe; during the battle of Magh Tuireadh he pierced Balor's evil eye with a magic stone."
—Devil Survivor 2 Compendium
Lugh is the demon that acts as the key to allow the Dragon Stream to take a different form. However, due to the Void's advance, his body is damaged, and a medium will be required to channel his energy. Based on the Demon Tamers' health screening results, Io is most suitable to be Lugh's medium. However, once Lugh is unsealed, his energy scatters across the map, and the party must gather all the pieces of his energy (or recover them from thieving demons) before demons can take them away.
When Lugh's energy is sent to Io, Yamato begins the ritual for the unsealing of the Dragon Stream, where Io is synchronized with Lugh. The ritual proves to be strenuous, and the player has to regularly heal Io and defend her from interfering demons. Once the ritual completes, Io unleashes the Dragon Stream on Mizar. Depending on whether the player chose to comfort Io prior to the ritual, the strain from channeling Lugh may end up taking Io's life.
If the protagonist does not support Daichi and Io is still alive by the 7th Day, Io channels Lugh during the fight with her and Daichi, complete with her race and Racial Skill changing to match Lugh's. After the player defeats her, Lugh is unlocked as a Unique demon for fusion in the Cathedral of Shadows. Lugh does not require a special combination of demons.
The Animation
Like in the game, Lugh is necessary to give form to the Dragon Stream in order to defeat Mizar. Io is chosen as the vessel for Lugh by activating Lugh's seal, Brionach. Once the seal is broken, Io is possessed by Lugh as her mind slowly overcome by the strain of Lugh's power. After Mizar is eaten by the dragon Shakko, Lugh takes full control of Io's body and shows its anger against Yamato, the descendant of Hotsuin family who had imprisoned and treating a god like a tool. Lugh ambushes the Diet Building to kill Yamato for revenge and effortlessly defeated JP's demons that guarding the building. However, Hibiki able to stop Lugh's attack by using Suzaku. Lugh defends itself from Baal's attack until it was defeated by both Suzaku and Byakko, leaving Hibiki and Yamato vulnerable. Realizing that Io's body won't hold on much longer, it launches its spear in hope to at least kill Yamato, but Hibiki pushes him out of the way and stops the spear with the Dragon Stream's magic circle. After much effort, Hibiki finally helps Io regaining control of her own body, breaking Lugh to pieces. Afterwards, Lugh is available as one of Io's demons.
Io summoned Lugh during the fight against Benetnach, but is sent back when Benetnasch uses its sound waves along with the rest of the demons.
Lugh is summoned again to fight against Nebiros along with Black Frost that fighting against Zaou Gongen when the path to Polaris is opened. Lugh holds Nebiros to allow Hibiki reaching the Transport Terminal until it was consumed by the Void. Afterwards, Lugh is sent to Hibiki's cell phone with other demons during the final battle with Yamato. Hibiki fused Lugh with Berserker, becoming Zouchouten.