Lilith

  WARNING ! This page contains  MAJOR SPOILERS . Read at your own risk.

Lilith was the youngest and only sister of the seven brothers, who died years ago before the events of the story. Lilith has never been seen in-game.

Personality
Little is known about Lilith's personailty. However, in the Devilgram Hide and Seek, Belphie remembers that she tends to get scared easily. It's also mentioned that once she became angry, it was hard to snap her out of it.

Background
Born as an angel in the Celestial Realm, Lilith was beloved by all her brothers. However, shared a special bond with Beelzebub and Belphegor. It's mostly unknown what her relationship was like with her other brothers. In the Devilgram Hatred, Mammon and Beel say that she was cute but troublesome to them.

Because of Belphegor's interest in humans, Lilith developed an interest in them as well and would also visit the human world. It was there that she fell in love with a human man, but kept the fact she was an angel a secret from him. Despite her efforts to keep her relationship a secret, Lucifer caught wind of the matter and went to the human world to see the man. To Lucifer, he appeared to be a good and virtuous man who made Lilith happy.

However, her beloved was struck with an incurable illness that Lilith sought to cure by giving him food from the Celestial Realm. This was strictly forbidden, as angels could not alter a human's lifespan. Because of her actions, she was sentenced with the ultimate punishment: to be obliterated. Her brothers would not allow that to happen, and their resistance was what caused the Great Celestial War. During battle, Lilith was gravely wounded and believed by most to have died from her injuries.

Instead, she survived long enough to enter the Devildom, but her life was hanging by a thread. After he arrived on the scene, Diavolo told Lucifer that he would risk interfering with the Celestial Realm's judgment by saving Lilith, but would only do so if Lucifer pledged his loyalty to him. Lucifer agreed, Diavolo then had Lilith reincarnated as a human who lived a happy and fulfilling life without any memories of her former life. Lucifer did not tell his brothers that Lilith still lived as a human due to a condition that they were not to contact her, and felt that he should carry the burden by himself.

After Lilith died and became a soul, she regained her memories of her brothers and her previous life in the Celestial Realm. Worried about her brothers, especially Belphegor, she traveled to the Devildom and remained there to watch over them, eventually forgetting how to return to the Celestial Realm. She waited for her descendant to find her so that she could lend them her power and aid her siblings. It is eventually revealed that MC is her distant descendant.

Main Character
While MC is a descendant of Lilith, neither they nor Lilith are genetically related to any of the seven brothers, by blood or otherwise. This is because angels are created, not born — none of the brothers are related by blood. The brothers choose to see their relationships to each other as just that: brothers. This is also why Satan is considered one of the seven "brothers" despite the history of his creation.

Trivia

 * Lilith was named after the succubus who was Adam's first wife, but became a demon after refusing to submit to him.