Ellie's mum is one of the characters we know the least about in The Last of Us' universe. We're going to be introduced to her in the TV show, but there was a time when she was going to appear in the original game, too.
In an interview with Variety, Neil Druckmann stated that he once wrote a story centred in The Last of Us' world that featured Ellie's mum. "It was supposed to be an animated short, but it fell apart and didn't come to be," Druckmann said. "There was a moment where we almost made it as DLC, but it fell apart."
In both games, Ellie's mother fails to appear, but she is mentioned from time to time by Marlene, who knew her before she passed away.
It's probably good for our souls that we never saw a story centred around Ellie's mother, as there's enough heartbreak in the world of The Last of Us as is.