Dís refers to any goddess of fate, spirit or ghost, benevolent or not, from Norse mythology. The English language does not consistently treat "dís" as a proper noun, hence the word may or may not be capitalized. The English also borrows its Old Norse plural "dísir."
"Goddess of fertility in Scandinavian mythology. Aesnir, Vanir and the Valkyries are usually considered Disen (plural of Dis)."
—Shin Megami Tensei III: Nocturne compendium
Dis first appears in the Assembly of Nihilo and can be recruited in random encounters. Her innate skill, Tetraja, becomes immediately useful to defend against the enemies in the vicinity who persistently use Death spells (including other Dis). After leveling her up at least twice, her Stone Gaze will be crucial in immobilizing the angels summoned by White Rider and Red Rider in their boss battles, preventing any new angels from being summoned again as well as wasting the enemy's turn icons while being petrified.
"The general term for a low-ranking goddess in Norse mythology, a kind of spirit. The Disir are said to have given birth to Valkyries, warrior maidens, and Norns, goddesses of fate, in response to the desires of humans for bountiful harvests and victory in battle."
—Shin Megami Tensei IMAGINE compendium
A variety of Dis can be found in the docks area of Shinjuku Babel. They appear alongside Thor in the boss room of Celu Tower's gold instance.
A trio of non-recruitable Disir appear in Sector Bootes. They used to possess time-based powers, until the rampaging Yggdrasil stole them for his own benefit and empowered himself as a powerful monster with full mastery above time, giving him the ultimate edge in battles. Desperate for help, the Disir encountered the protagonist and convinced him to attempt to destroy Yggdrasil, teleporting him to Sector Eridanus.
The Disir were, however, confused to see a second version of the protagonist pop up just as soon as the first one had been nearly killed by the rampaging tree; at this time, the sisters' combined future form, Norn, canceled the stolen powers, removing Yggdrasil's edge and leaving him open for a counterattack. Realizing they were meant to eventually reach the power of Norn and send the protagonist into the past with their restored powers, they sent back the past protagonist to Bootes with no memory of the fight and rewarded him, instructing him to remember he still has yet to destroy Yggdrasil for good before the ontological paradox was completed and the loop closed.
A non-hostile Dis appears in the Challenge Quest, The Only Food Source, after Tlazolteotl has been defeated. She reveals that demons had been sent by the Gods to kill humans and revealed Tlazolteotl's past and her intentions behind keeping the corn. Dis' job was to transport the souls purified by her and asks Flynn to treasure the memories of the dead and the gift from Tlazolteotl.
"The general term for low-ranking deities in Norse mythology or a kind of spirit. They are said to bring good luck in battle and also represent fate, fertility, and good harvest. Some view them as the servants of the Valkyrie."