In Season 4, Episode 17, “Because the Night,” the show looks back at Damon’s (Ian Somerhalder) life in New York City during the 1970s. As seen in promo photos, he had a surprising companion: Lexi, Stefan’s deceased best friend.
In an interview with Entertainment Tonight, Ian explained how those two ended up together. “What's cool about it is that you see how long and turbulent their history is,” he says. “There are just so many different agendas and egos at play, but at the end of the day, both Lexi and Damon love Stefan, so to see them interacting throughout an episode where they're not trying to kill each other or making derogatory jokes to one another the entire time is amazing. There was a real relationship there, so it was fun to explore.”
While that love for Stefan was enough to make them at the very least civil for a time being, it wasn’t enough to keep Damon from killing Lexi in Season 1. Ian says, “I think if you go back to that episode after seeing next week's, the look of betrayal on her face, the shock of it all, is recontextualized. Which I think is a great way to elaborate or, at times, strengthen or fix a storyline that maybe didn't work as well. It's an added bonus to the structure of how our show works.”
This makes retroactively mad at Season 1 Damon all over again!