I decided to test this myself in Free Training just now, and it turns out that the non-Representing Character actually doesn't influence your Attack or Defense stats.
Here's the video I recorded as proof: https://www.youtube.com/watch?v=ZhfhMZXwgyg
Topic on Talk:Lu/Ciel
From Elwiki
Yes, this was changed a while ago. Only the HP acts like that afaik.
The reason I brought it up is because the recent edit to Tips and Details says that the non-Representing character does influence the Attack and Defense stats.
Also, before I saw your reply, I did another test, this time to see if they shared their Attack/Defense at all. Here's the video of it: https://www.youtube.com/watch?v=waymj6Rc54I
