"The childhood name of Cú Chulainn, a clever Irish hero of Celtic mythology. He wielded the holy spear Brionac and was the son of the sun god Lugh, of the Tuatha De Danann. Some suggest that he was a real historical figure. His mother was Deichtine, sister of the human king Conchobar, who doted on him as a child. When Setanta was seven, the king was invited to a party by Culann the smith. On his way, the king spotted Setanta playing at hurling. Impressed by the boy's throwing performance, he invites Setanta to come to the party as a reward. Setanta promises to come later, but the king forgets he had invited the boy, and Culann releases his guard dog into the garden to ward off intruders. Unaware of this, Setanta arrives at the party and is attacked by the dog, but chokes it to death. Seeing this, Culann the smith is beside himself with grief, but Setanta says to him, "I shall become your guard dog." From that day forward, Setanta was known as "Cú Chulainn", (Hound of Culann)."
—Shin Megami Tensei IMAGINE compendium
Setanta can only be obtained through fusion and appears in a central role in the Foreign Country in the Woods questline. Players can obtain his scarf and gear through fortune cards.
Setanta appears in the Challenge Quest, Cu Chulainn's Training, where Cu Chulainn has been turned into Setanta by his master Scathach. She requests that Flynn teach Setanta the "way of the warrior" in the Bugeikan stadium. Arrogant, he proclaims that killing them will be easier than killing the guard dog as the battle starts. He is surprised when he loses, and realizes the lesson his master wished to teach him by fighting Flynn, and is returned to his true form, Cu Chulainn.
Setanta later appears in Shibuya. He can teach Flynn the Charge and Javelin Rain skills through his Demon Whisper.
"A brave young man in Celtic myth. After defeating a vicious dog with his bare hands, he took up its role as guardian, and gave himself a new name: "Cu Chulainn," meaning "Hound of Chulainn.""
—Soul Hackers 3DS compendium
Setanta is a demon exclusive to the 3DS remake, available after Nemechi evolves into the leg form (female growth path), and costs 150 D-Souls. Unusually, despite being Cu Chulainn's younger form, he is actually at a higher level.
"A brave young man in Celtic myth. After defeating a fierce guard dog, he volunteered to take its place, earning him the nickname "Hound of Culann"."
—Persona 5 background
Setanta is the third Persona of the Emperor Arcana and can only be obtained through fusion in the Velvet Room. He is one of three Personas to learn the Dormin Rush skill. When itemized through Electric Chair execution, Setanta yields the Counter skill card.
The Shadow of Nozomi Odo will also take the form of Setanta when confronted in Mementos during the "Part-time Job, Full-time Hell" request.
"A brave young hero in Celtic lore. He was attacked by a vicious guard dog when he arrived at Chulainn manor, killing it with his bare hands. He took up its role as guardian, and adopted the name "Cu Chulainn", meaning "Culann's Hound" and swore to serve and protect until a placement was sired."