When Kingdom Hearts: Chain of Memories was announced, it seemed to be a bit of a joke. Everyone knew that Kingdom Hearts was to have a different sequel, Kingdom Hearts II, on the same platform as the original. Typically, there will be Series Name, then Series Name II, Series Name III and forever up to Final Fantasy XII, with no room for a Kingdom Hearts 1.5. So where does that leave Chain of Memories?
It has the same protagonist, same main characters, same worlds, same monsters, and a similar premise to the original Kingdom Hearts. It takes place immediately after the first game, under the assumption that all the characters have lost their memories, and are trying to recover them (by playing through all the same worlds all over again).
At first, I thought it was essentially Kingdom Hearts pocket-sized, so that I might carry it around in my GBA and play it on the go. But the battle system changes gameplay entirely.
Kingdom Hearts was a Square-Enix RPG that worked well in real time. The graphics flowed nicely, there were no transitions into battle mode, and if it was turn-based, it would have felt too much like Final Fantasy with Mickey Mouse. Chain of Memories keeps the real time system, but adds in the battle transitions.
Still, Riku has to fight his replica during the game too. As for battling itself, he doesn't have to reload his cards after going through the deck; hooray! Riku only has one summon, King Mickey, which is much better than any of Sora's. All in all, Riku is a much cooler, and less awkward, kid to be, despite the attitude problems.
As music and visuals go, it's hardly an improvement. The music is slightly different, but it feels more or less the same. As in the first, there is one general battle music theme and one general walking about song for each world.
The graphics are as good as you expect from a Gameboy Advance game, but it is significantly more pixilated than Kingdom Hearts. Sora has been reduced to a little sprite, along with all his friends and enemies, aside from very emotional headshots that supplement dialogue (or the FMV equivalent of Chain of Memories, very similar to the silent, word-bubble FMVs that appeared occasionally in the original).
Overall, Kingdom Hearts: Chain of Memories is decent supplement to the series that fills in holes in the plot, rather than building onto it. It serves its purpose, in a convenient and portable package that is uniquely entertaining. If you're a lover of the series and care about the plot, then it's worth buying. As Gameboy Advance role-playing games go, this is certainly one of the better ones.