Loki

Loki (aka The Zodiac Fanatic) is the primary antagonist of The Zodiac and is one of the obstacles during Lara Croft’s mission to find the thirteenth zodiac.

Discovery of the Astronomy Tower
As explained, Loki is the remaining (including Lara) hunters for the thirteenth zodiac. All other stalkers have either been killed or have given up.

Hunting in Tibet
After Lara’s escape, Loki begins her quest to find the three keys responsible for unlocking the tower. However, her plan foils after Lara collects them and is mislead to the other side of Tibet. This is so that Croft could make it back without being seen.

It is not known why Loki wants the zodiac, but is recognised that one of the family members may have mentioned it.

Death
During the fight, she disarms Lara. Croft manages to retrieve her weapons and push Loki out the circular window, leading to her death. After, she is thrown into the river by Lara and found at Scandinavia. She is cremated by her family in a nearby cemetery which would leave her plans in complete vain.

Trivia

 * Loki is a Marvel villain and also a Norse god.
 * It is not unanticipated that her alias is The Zodiac Fanatic because she is very obsessed with it.