When Advent Rising's protagonist, Gideon Wyeth, and either his brother Ethan or fiancée Olivia Morgan (the player must choose which to save in the previous level), need to get the hell out of Dodge (Dodge in this case being the planet Edumea, which was under attack from the human-hating Seekers), the ship they end up leaving on is the one piloted by Marin Steel, one of the best starship pilots known to man. Steel then accompanies Gideon and company as they flee the Seekers, and becomes quite close to Gideon as he unlocks the Human Power hidden within him.
By the end of the game, Marin is Gideon's last remaining friend; whichever loved one he chose to save on the space station ends up drowning in a crashing starship, and the aliens who seek to protect him from the Seekers end up being killed by the brutal race. In the final battle of the game, Gideon and Marin are faced with a "true human", in the form of either Ethan or Olivia, whoever was left behind in the first mission, having been genetically and cybernetically upgraded by the Seekers for some nefarious purpose. Marin by this point has managed to unlock a little Human Power of her own, and aids Gideon by shooting the occasional fireball at the true human, but he'll still have to do the heavy lifting. Marin is not drawn into the portal created when the creature is killed, and other than it being heavily implied that she would go on to become Gideon's love interest in subsequent games in the Advent Trilogy (presumably once he had gotten over Olivia's death), it is unknown what was meant to happen to her.
In addition to her role in Advent Rising, Marin was slated to be the protagonist of the PSP side-story Advent Shadow, which would have been a prequel showing Marin's career as a mercenary and pilot-for-hire in the days before the Seekers destroyed Edumea and she met Gideon Wyeth.
Unfortunately, Advent Rising was a critical disaster, and the other two games in the Advent Trilogy were scrapped before their titles were even announced, along with Advent Shadow, so it seems Marin Steel's story will never be finished.