Being a fellow fourth year, Addison and Gideon would share most of their classes together. Now, I have no idea how much Gideon would pay attention to Addison.
She's a bookworm herself, and a perfectionist. She would be a regular in the library as well, and will excessively study. Any work that she completes, and any partner work in class for that matter, would have to meet her perfect standards in order for her to be completely okay with handing it in. However, she doesn't spend her entire time studying. True, she would normally finish her work first, but she does do other things than study. She likes art, and would freely wander the castle looking for inspiration. Being curious, she wouldn't mind walking into forbidden locations just to see what was on the other side. In addition, she is loyal to her friends and would accompany her friends to anything dangerous to help them if need be. She enjoys being outside, and likes flying and Qudditch, in addition to muggle sports as well.
To basically sum up my giant paragraph above, Addison and Gideon seem to be alike in so many ways. In any relationship that they have, Addison would likely encourage Gideon to at least not hate his brother, and use her own relationships with her two brothers as an example. Oh, and Addison loves Herbology, if that helps Gideon or not.