1. Who they are
Sara Lance (White Canary) — Co-captain of the Waverider. Bisexual. Died at least twice in canon and was resurrected each time, first by the Lazarus Pit. Trained by the League of Assassins. Survivor of Lian Yu island (twice), multiple wars across time, alien abduction. Physically covered in scars. Runs on chaos and competence; the two are the same thing in her body. Previous significant relationships: Oliver Queen, Nyssa al Ghul, Leonard Snart. Sister Laurel Lance is dead. Father Quentin Lance died in Season 6. Her whole family is mostly dead and her found family is the Legends.
Ava Sharpe — The twelfth genetically engineered Advanced Variant Automation clone, manufactured in 2213 by Rip Hunter for the Time Bureau. Has implanted false memories of a normal childhood and fake parents who don't know what she is. Former Director of the Time Bureau; later joins the Legends. Highly competent, by-the-book, deeply uncomfortable with chaos. Her central wound: not believing she is real, not believing she is a person, not believing anyone would choose her specifically when she is technically replaceable.
The dynamic: adversaries-to-lovers who fell in love before they liked each other. Ava thought the Legends were reckless idiots; Sara thought Ava was a bureaucratic nightmare. Both were right. Both were also exactly what the other needed. They are the most stable and domestic of the Arrowverse couples; they are also the ones with the most existential damage.
Canon endpoint: Married. Expecting a child (via Kriblix DNA, the show's solution). Had planned to retire before being pulled back in. The fandom's job is to fill in the years the show didn't show.