Petrine is one of the Four Riders of Daein that appear in Fire Emblem: Path of Radiance. The Four Riders are the four most powerful officers under the command of King Ashnard, and of them, Petrine is among those seen most often in the game. She is a paladin-class unit and wields a spear imbued with elemental fire. Early in the game, she is instructed by Ashnard to hunt down and capture Princess Elincia and recover Lehran's Medallion. Among those under her command are Ena, whom Ashnard assigns to Petrine as a strategist, and Shiharam, Jill's father.
In terms of personality, Petrine is confident in her abilities as a warrior and disdains incompetence. She at one point takes on Greil in a fight, only for it to end in a draw. The only person to whom she shows any outward sign of fear is Ashnard.
Her mission is finally cut short in Chapter 23, where she is killed on Riven Bridge. Despite her confidence and bravado, these traits vanish when she is mortally wounded. It is revealed in her panic that for all of her tough talk, she is terrified of dying. If Soren faces her in battle, she will recognize him as a fellow Branded, but he in turn sees through her bravado and states that he will show her "true fear."
Heritage and Racism
One notable trait of Petrine is her strong racism against the laguz. This can likely be traced back to her identity as a Branded, which the laguz hold a great contempt for. Among the laguz, it is generally believed that romance between laguz and beorc is a sinful abomination.
This history is expanded upon more in the following game, Fire Emblem: Radiant Dawn, in which it is explained that laguz refer to the Branded as "Parentless" and, when in the presence of a Branded, act as though the individual does not exist. This attitude of the laguz toward her kind may explain Petrine's vitriolic hatred of them and her support for King Ashnard despite her utter fear of him.
As a Player Character
Petrine can be unlocked for use on the challenge maps in Path of Radiance after the player has beaten the game seven times. As a player character, she is no different than a standard paladin unit, but comes equipped with her flame lance.