Isabel was a member of the Maximillian Knights. As such, she has a strong sense of justice and stopping evil doers. After the Sun Rune War, she and Mathias traveled to New Armes. With King Jalat's permission the two went on a top secret to find and root out any corrupt nobles that still might be in support of Ayers or desire to do anything else that bring harm to the people of New Armes.
When Jalat was murdered, Isabel joined Yvaine's army in an effort to bring those murders and traitors to justice.
To avoid more spoilers, this is all the info to be posted for now