Dagda is the supreme god in Celtic mythology. His name means "the good god", good as in good at everything, as opposed to morally good. He is portrayed as being armed with a magical club. During the battle against the Irish aboriginal, Fomorians, Dagda is injured and begins to live a reclusive life. His positions are succeeded by his brother Nuada and daughter Brigid.
A demon who meets Nanashi in the underworld. He informs Nanashi that he is currently in Yomotsu-Hirasaka, and states that he has died, and that his friend, Asahi, would soon follow him. Dagda claims that Nanashi has accomplished nothing in his short life, but that together the two of them could change that. Dagda says that he will revive Nanashi, if he agrees to work with him. Nanashi agrees, and Dagda revives him, leaving behind green markings on his left hand, and Nanashi's eyes turn green. Dagda states that he is now his personal Godslayer, and will commit "deicide" as his puppet, though he later says that Nanashi doesn't necessarily have to be obedient to him, except if the player makes certain choices during the game Dagda will seize control of Nanashi's body, saying that Nanashi is nothing more than his puppet, and force a certain choice upon him, regardless of the player's wishes. After resurrecting the young cadet, Dagda enters his smartphone to observe him during his battle against Adramelech, repairing the smartphone to allow him to summon and recruit demons to his aid.
Through out the course of Nanashi's journey, the player learns that Dagda's ultimate goal is to kill all Gods and Demons, believing that divine power had corrupted them, and he would be no exception. To that end, he needed a pawn, that being Nanashi, who could destroy them, as a Godslayer has the power to kill all godly beings, unlike other entities who can only seal one another away.
In the Peace Route, after Nanashi states that he will destroy the Cosmic Egg, Dagda chooses to take back the life he granted to Nanashi, telling him he can regret his decision after being on the brink of death once again. For the first few turns, Nanashi's life will drain away, and his demons won't listen to his commands. Danu, however, saves Nanashi by re-creating Dagda, in order to sustain Nanashi's life. Danu implies that the new Dagda is made from the old Dagda's true feelings. Dagda is defeated, but rather than dying, he disappears, likely fading away from existence and allowing the new identity to take his place. The new Dagda reveals he shares the old one's cynical and misanthropic nature, but is not so focused on his own goals he is not willing to accept compromise or his mother's advice.
In the Anarchy Ending, Dagda succeeds in his plans, as Nanashi has slain YHVH at long last, albeit at the lives of his friends, and many others. Regardless, he is capable of replacing YHVH, though he passes this ideal to Nanashi, who is human, thus he can always remember the consequences of being corrupted and becoming arrogant. He later disappears after the creation of the New World Nanashi establishes.
Dagda is a dark element unit obtained through a twitter event leading up to the Shin Megami Tensei IV Final x Brave Frontier event. He was released to all players after surpassing the goal of 4000 tweets.