Reks was an incredibly minor character. And not all FF characters are meant to look like heros. Just look at Bartz, Squall, Zidane, and Tidus.
I would personally choose Ashe because she was the real main character of FFXII. The other characters only had what I would call side stories.
Having Ashe in the game would mean that Vayne would have to be her opposing villain. Basch would have to oppose Gabranth and Balthier would have to oppose Dr. Cid. (Vaan, Penelo, and Fran don't have any obvious opposing villains.)