At first I thought he was going to be a cute little pet, but he is a full member of the ka-tet, at least in my eyes.
The way he helped Roland rescue Jake just rocks!
He's obviously more than a simple animal -- Blaine names him as a member of the ka-tet, and as the ka-tet approaches the green glass building in Kansas, they just found that Oy was left red shoes, too, along with the other members of the group.
I realize he will probably come to a tragic end (from Roland's vision) -- but please don't spoil me, since I am only nearing the end of Wizard and Glass.
)