May was the daughter of Lily Sunder.
Born sometime before the 20th century, May was Lily Sunder's daughter when she was still a professor of apocalyptic lore. Lily's studies had led her to summon the angel Ishim, who developed an obsessive passion for her.
Lily realized this angel was the equivalent of a monster, and chose to cut all ties with him. When Lily married Akobel, a jealous and enraged Ishim tricked his garrison into believing that Akobel had broken the rules by fathering a Nephilim. Akobel was executed, while May was murdered by Ishim in retaliation for her mother's marriage to Akobel.
The other angels never found out that the child was a normal human, or that Akobel's charges were based on Ishim's jealousy. In 2017, Lily sought revenge for the murder of her daughter and husband, killing Benjamin, Mirabel and two unnamed angels who took part in the murders. When she attempted to kill Ishim and Castiel, the only two angels remaining, Lily came into conflict with the Winchesters and explained the truth to them. Believing her story, the Winchesters aided Lily in getting revenge against Ishim. Ultimately, Ishim was killed by Castiel, the last of the angels who took part in May and Akobel's murders. Afterwards, Lily told the Winchesters she wasn't sure if she could give up on revenge as it was all she'd had for over a hundred years, leading Castiel to apologize for his role in May and Akobel's deaths. Castiel promised that if Lily couldn't find it in her to forgive him, she could return at a later date to get her revenge.
After getting her revenge, Lily sought to preserve the little that was left of her soul so that she could be reunited with May in Heaven. As a result, Lily gave up her Enochian magic and began aging. When the Winchesters called upon her to help save the Nephilim Jack, Lily explained her desire to reunite with May and asked for their help in ensuring she gets into Heaven in exchange for her help in saving Jack. After Anubis reveals that Lily will go to Hell, she initially refuses to help until Dean asks her to help them so that they will not have to suffer the same kind of pain Lily suffered by losing May.
After helping to save Jack, Lily suffers a heart attack and dies. In the afterlife, Anubis judges her soul once again and determines that Lily can go to Heaven. Lily is finally reunited with her daughter in the afterlife and Anubis asks Lily to tell May that he says hello.
- May owned a doll given to her by her father. Akobel found the doll disturbing.
- Anubis' comment to tell May hello from him suggests that he knows May personally or at least has met her.
- May is Lily's primary motive in all of her actions: at first, her life is dedicated to getting revenge for May's murder. Once she has that revenge, she's dedicated to reuniting with her in Heaven.
- Though not outright stated, Lily appears to have forgiven Castiel for his role in May's death as she shows no animosity towards him when they meet again in Season 14's Byzantium.