as the star of his own game

Many game protagonists are silent and withdrawn to begin with. This allows the player to bring the protagonist to life through their own choices, which in turn makes the gameplay more individualized. Some game series perform this task better than others, for example the Persona games leave their protagonist completely open while Tales of... games have often very opinionated heroes. Final Fantasy games often lie somewhere in between. Cloud, Vincent, and your TURK of choice tend to lean towards the silent protagonist. Zack however brings a completely different view, as an open and outgoing character he often makes crazy choices, which limits the player's customizations to be made in his battle style.

While in most games, this would be an almost unwanted characteristic in the opinion of most fans but Crisis Core is not really your average game. Crisis Core was never meant to stand on its own, and instead it simply expands on an already existing storyline. Therefore, the player's choices don't matter. Regardless of what the player does, the game will still come to an end and Zack will still die The only difference is in how long it takes before he dies and how many side missions you take up. Because your play style doesn't matter, and your personality doesn't matter the game is very two dimensional. You merely help Zack along in his missions.

Like I said, in most games this would be a bad thing; however Crisis Core plays like a story. You can read it from start to finish but how much you take out of it is completely up to you. At the same time, Crisis Core isn't bogged down with cut scenes like some games (read: Dirge of Cerberus) which makes it easier to get into. You can get a lot of enjoyment out of playing it but you could never play it as a standalone game. It just isn't long enough for that.