cause the Templars ordered her to isn't anything more than that. That is where it also differs a bit depending on your path. On the love path she fought against the order and only became the MC's GF later when she wanted if I recall correctly.I ended up on her "friends" path on my first playthrough and it's forever tainted her as a LI for me.
If she hadn't started out as the MC's girlfriend then she would be much more tolerable. I don't see why she had to fake a relationship with him when they were already close friends.