This game is fun!

First thing, I'm not a big Kingdom Hearts player. In fact, with the exception of the time I played the first 3 hours of Kingdom Hearts 1 on an emulator, this was my first experience of a KH game as a player. I have to specify "as a player" because I used to watch KH let's plays when I was younger, so I'm kinda familiar with the story and characters.

For those who don't know Kingdom Hearts, it's a series of action RPGs featuring characters from Square Enix's games (particularily from Final Fantasy) and the Disney movies. While it sounds like a strange combination, I would say it works pretty well in the narrative that has been developped around it, which is very "Light VS Darkness" driven. But let me introduce the game I played because this is not a review of KH story as a whole.

In KH: Birth by Sleep, you follow the story of 3 friends: Ventus, Terra and Aqua. The three of them are training to become Keyblade Masters, wielders of swords named Keyblades, that have the power of unlocking hearts and opening pathes to other worlds. The game is structured in an original way: the 3 friends will be separated at the start of the game, and you will play each of their stories in any order you like, trying to piece together the complete story.
While the game lets you pick who you want to play first, there is an optimal order that the game gives you (it is Terra, then Ventus and finally Aqua). I chose not to listen to this suggestion and played in this order: Ventus, then Terra and finally Aqua. I don't think it matters too much but you should know that some of the intial exposure happens only in Terra's story, and the final chapter that you can unlock after beating all stories takes place from Aqua's side of view.

That being said, if you are not familliar with KH I would not recommend playing this game only for the story. It serves as a prequel to explain stuff that happens in KH1, 2 and probably 3. If you don't know either of those games, a third of the story elements brought in the plot will remain unexplained.
Moreover, while the story concepts are great and have potential, I found that the writing was pretty bad. The characters are almost every time either very good or very evil (most of them being Disney characters, it makes sense, but it is kinda boring on the long run), plus they have a tendancy to repeat themselves a lot and the usage of many of the Disney worlds seems uninspired. It's a shame considering the unique mix of Square x Disney has a lot potential to create some great scenarios. And it happens, sometimes. But there are also many moments when it feels like you are watching a Wish adaptation of the movie, with some characters that do not belong here. It is underwhelming to have the most interesting parts of the game story-wise not having anything to do with the Disney worlds directly.

The poor writing is a shame because otherwise the game looks amazing for being on a PSP, the animations are very cartoony and expressive and there is some cool scenography in some cutscenes.

I think I made my point on the story of the game now.
And after reading all of that, you must be wondering why I still put 4 stars on my review. And that, is because of the gameplay!

The 3 characters controls are about the same, with slight variations in movement speed, jump height and dash. Also, Terra specialize in physical attacks, Aqua in magic and Ventus is the middle ground.
The movement and combat system is very slow and not really satisfying at the start, but this quickly evolves as you unlock new skills and abilities like dash, parry, counter, etc... To fight your enemies, you have many possibilities:
First, you can swing your Keyblade to deal damage to an enemy close to you. You are allowed to perform a small combo, both on ground and in the air. Your second option is something called a "Command Deck". This deck is basically where you put your skills, that can be magical or physical. Each of these skills can be used at any time if in the deck. Once used, they are put on a short cooldown to prevent you from spamming them. Your deck starts with 3 slots, but rapidelly increase in size as you progress.
Attacking enemies with thoses two types of attacks allows you to fill up a gauge, which quickly runs out if you spend too much time not attacking. This gauge is what brings this battle system all its fun! When filled up, depending on what type of attacks you have used, you will activate a "Command Style". This changes your combos for better ones for a limited time, while also giving your attacks elemental powers or extended reach. And then, as you continue attacking, you can fill up the gauge once more to either activate second or even third "Command Style", or activate a devastating attack corresponding with the command style you are currently in.
This is what makes the game fun to me. Trying to always be attacking an enemy so that the gauge stays filled up, and managing my skills to activate the right "Command Style" that leads to the best combos.
Also, I have to mention that your strategy is constantly evloving. This is because your skills can level up. There are between 3 and 5 levels per skills, and they level up if they are in your deck, even if you don't use them. When a skill levels up it becomes stronger, but more importantly, when it reaches its max level you can fuse it with another skill to create a new one. This allows you to mix a Physical and a Magical skill together for exemple, to create a physical attack with elemental properties.
And what pushes you to change your deck as often as you can is this: every skill that is created by mixing two other skills will have a small passive boost that is active if the skill is in your deck. BUT, if you are able to level this skill to its max level, the passive boost will always be applied to your character, even if your remove the skill or fuse it with another one.
I think this system is brilliant because it rewards you for changing your play style very often and pushes you to improvise a lot during combat. This means that the regular enemy encounters are not as repetitive as they could be without it.
(There are also summons that allows you to access the moveset of people your character created bonds with, but I didn't use them that much.)

What's more, the game manages to have a decent camera despite the absence of a second stick to control it (instead you use the L and R triggers to rotate the camera around your character). You will sometimes get hurt by an enemy offscreen but most of the time you have a good view of all enemies, or the enemies will be numerous but mostly passive.

That being said, I have to talk about the difficulty. When you start the game, you must choose a difficulty, between "Story", "Standard", "Hard" and "Proud", and you will be locked in that difficulty until the end of the story you are playing. I played Ventus story on "Standard". It was fine, but outside of bosses I felt the game was too easy and I didn't need to use it's systems. That's why I switched to "Hard" difficulty for Terra's and Aqua's stories. While I overall enjoyed the game much more, I encountered severe difficulty spikes.
The game does not give you character invulnerability frames when getting hurt, so if there are many enemies around you, life can go down very quickly. Sometimes it almost feels bullshit when you die in 2 sec at full health. Also, the parry, which is another important part of the battle system, feels very slow and unresponsive. I've had multiple occasions where I saw an attack coming, pressed the parry button but nothing happened and I got killed. This can be a frustrating, especially if you are under leveled. This is more true at the end of Terra's story and at the beginning of Aqua's.

Last thing, the soundtrack is great but I which there were more tracks, because there is only one music for exploration per world and it gets very very repetitive (I still have the Cinderella track playing in my head, help).

So, if what I described sounded interesting to you, I recommend it. Each character story takes around 8 to 10 hours to complete and you can take breaks between them if you want so it is not a very time-consuming game. But if you want to unlock every secrets, then you will have to grind for hours and play horrible mini-games (fun).
As for myself, I am curious to play other KH games, maybe the mainline ones, to see if the gameplay is as fun as in this one and if the writing gets better when it's not a prequel.

Thanks for reading my review!