At first, when she's introduced it's shown pretty clear that she disdains the Laguz species, heck, the only reason why she was recruited was because she noticed Ike's ship was being attacked by a Raven tribe of Laguz. At this time, she may seem like a one dimensional character, but then a little later in the game, after being around Ike's army, which has Laguz like Lethe and Mordecai in it, she opens up. She realizes that not all Laguz are "scum" like she was taught and raised like in Daein. This can even be seen in her supports with Lethe, with at first the two hated each other, and soon they realize they are not so different after all, and become close friends. This is not the only thing that describes Jill also. It shows many times in the game how she cares for her father who is one of the generals of Daein. Soon in one chapter you actually have to fight her father, and there's no choice, but to kill him. Interestingly enough if you choose to bring Jill in this chapter and have her fight her father, there's a chance she might actually switch sides and defect! (Yet, it's not recommended to do that) Anyways, after you beat the chapter, without that happening, Ike and Jill have a conversation. Ike states that he's willing to let Jill leave if she wants to, even going as far as saying that if you want to avenge your father, you can me. (Very much like the conversation with Robin and Lucina) Yet, Jill decides to stay and help the cause. She's one of my favorite Fire Emblem units because of how deeply written she is. This is also why Robin and Lucina are the only interesting units in Awakening