Yes, Chain of Memories actually has two story modes, one starring Sora and one starring Riku, which is unlocked after completing “Sora’s Story.” The GBA game even has two separate save files for Riku mode games, to match the two it gave Sora. (Not the case in the remake! Make sure not to save over your Sora clear data in the remake!) Riku’s mode is called “Reverse/Rebirth” (the Japanese words are homophones), with “Reverse” as a reference to the term from Tarot, not to mention a few other metaphors. And yes, it is a more-or-less a full game, or at least it contains a full twelve floors: the same floors as Sora’s Story, minus 100 Acre Wood.
From a Retrospective perspective, R/R is a lot easier to cover, since you know all the basics and the story is limited for reasons you’ll see in a moment. Still, it’s exciting to play as Riku for the first time, and it’s also faster to play through than Sora’s Story, so it drags less than you’d expect (though that may not be much comfort if you thought the game was already done!). I find I can do each world in under half an hour and not feel like I left much behind, while Sora’s story was closer to a full hour. The design even seems to encourage rapid play. Why, it’s almost as though the developers realized a full-length portable game had already gone by and they were just grasping at straws! One of the major appeals of R/R on the GBA was that it was a challenge mode, for a variety of reasons you’ll see as we go, but the remake removed the “challenge mode” angle, realizing CoM’s gameplay was not particularly popular but the story still very important. They… probably could have done more to create a fluid experience.