Steph was originally a character in the 2013 reboot of Tomb Raider. However her character was cut from the story in final release. She was a colleague of Lara's aboard the Endurance. Unfortunately, she became the first victim of the scavenger in the Scavenger's Den.
No official backstory was released, but she was mentioned several times in the demo and the trailer "Crossroads". Lara finds her body hung up in some sort of ritual and exclaims "Oh God, Steph!" Then, after the rest of the Endurance crew pulls her out of the bear trap, she immediately tells them that Sam has been taken by a man: "...Like the one who killed Steph." The rest of the crew is shocked to hear of Sam's kidnapping and Steph's death.
In the final game, A woman's body is still encountered in the Scavenger's Den, but Lara never refers to the body by Steph's name, saying instead "Oh God, what happened to you?" showing how her character was cut from the game.
Steph character is still available to use in multiplayer mode upon becoming Level 24, and she is included in a piece of concept art depicting several Endurance crew members.