Cynthia LaFave had a word of warning when she first met T Kira Madden in 2015.

“She said, ‘If you hurt my daughter, I’ll kill you,’” Ms. Madden recalled.

And that, by Ms. Madden’s reckoning, was a fair enough thing for her to say about her relationship with Hannah Beresford. Years earlier, Ms. Beresford had fought an episode of depression so crippling she required hospitalization.

Ms. Madden was no stranger to pain, either: Her 2019 memoir, “Long Live the Tribe of Fatherless Girls,” outlines her trauma-filled coming-of-age as the queer, biracial daughter of a pair of well-to-do addicts in South Florida. That Ms. Madden’s pain may have affected Ms. Beresford was a reasonable concern for her mother.

It proved unwarranted. “Their relationship has brought so much peace to them both that, as it stands now, if anyone tries to hurt Kira, I’ll kill them, too,” Ms. LaFave said.