Seraphim are angels with many wings in Judeo-Christian mythology. Their name means "burning ones", indeed seraphim are usually depicted as angels as fire and are described as being aflame with the love of God. Satan, or Lucifer was said to have been a seraph sporting 12 wings before he fell.
"The highest rank of all the angels, it is the being closest to God. Humans can rarely detect their presence, but they are said to have four heads and six wings. They also appear in human form."
—Shin Megami Tensei: Strange Journey Compendium
Seraph is an extremely high-level demon found in the depths of Sector Horologium, created by a ritual carried out by eight angels, one from each angelic rank, from Angel to Cherub, fusing themselves together to create the massive holy being. Its relation with the Protagonist varies depending upon the selected path: if the Protagonist has chosen the Law route, it will inform of the return of its enemy, the monstruous Mara. It will then post an EX Mission calling for Mara's extermination. Upon Mara's death, it will meet with the Protagonist and request to join his party. This will also unlock the special Seraph Fusion, which, like the ritual, requires an Angel, an Archangel, a Principality, a Power, a Virtue, a Dominion, a Throne, and a Cherub.
In the Chaos route, the Protagonist instead meets with Mara first in Sector Grus, and receive an EX Mission to kill the angels before they combine into Seraph. If he attempts to stop the fusion, he is swatted away and Seraph comes into existence anyway. Upon its defeat, Mara appears and will try to join the Protagonist.
Seraph appears as a boss on the Chaos and Neutral routes, where it is one of the demons creating a barrier that prevents further entry into Purgatorium. They reside in the 5th Crown. Unlike most other angels they do not repeat the same words, instead stating that they will not have to involve God's Chariot in killing them and that creating their perfect world requires that not even a speck of filth remain. In battle Seraph uses the Light Wing skill, which reflects all damage that doesn't match the color Light Wing takes.
During New Game Plus a Challenge Quest called Four Wings, Four Heads can be done on Jonathan's Route. Aniel requests that four demons be slain so that their heads will be offered in the sacrifice needed to summon Seraph. By calling the name of the Four Saints the four heads are sacrificed, summoning Seraph. Seraph introduces itself as the Burning Flame, remaining silent afterwards but radiating light.