There's a timeline where a third member joined Elizabeth and Booker, her name was Vivian Monroe who wanted to escape Colombia seeing how bigoted it was. Elizabeth and Vivian later fell in love with Booker being oblivious to it thinking they're good friends. When the whole dimensional purge happened, the two alongside Booker survived and ended up living in the modern era. Booker is still oblivious.
Booker do be an oblivious man. “A city under the ocean? Impossible.” Booker. You are literally in a flying city, my guy.
