Virmire is a planet on ME1 where the player must choose to kill off a squad mate: Ashley Williams or Kaidan Alenko. Ashley is a female solider while Kaidan is a male sentinel. Both are potential romance options for a hetro-Shepard.
In ME2, after the Horizon mission, Shepard has the opportunity to speak to the Virmire survivor:
The former squadmate is highly upset about Shepard's alliance with
Cerberus. Williams or Alenko question Shepard's loyalty and refuses to
join Shepard's mission. From the conversation, you learn about what
has happened over the last two years and what Shepard missed while
being on an operating table.
PC Gamer and various other sources have confirmed that the Virmire survivor (Ashley or Kaidan) will be a squad member. According to Playstation 3 Magazine, UK Edition, June 2011, Issue 140, the Virmire survivor has become a spectre. Ashley can also be seen at the end of ME3 extended walkthrough from E3.
Here are the ME3 portraits of Ashley and Kaidan: