Aya is not all that skilled in combat, but because she's the only person capable of using Overdive, she's been tasked with wiping out the Twisted, strange creatures who've invaded Manhatten. Using Overdive, Aya turns herself into a spiritual existence that can board and take control of the bodies of other people. This plays a major role both in the game's story and in its battle system.
During battle, Aya uses Overdive to take over the bodies of the soldiers who have spread across Manhattan in the fight against the Twisted. Doing this gives you control of the soldier. You basically become the soldier, inheriting his life and weapons -- although it still looks like you're controlling Aya.
There's no limit to how often you can execute your Overdive move. One suggested use for Overdive is to switch off from one soldier to another, positioning everyone in order to surround enemies.
In one set of screens in the Famitsu.com story, you can see Aya, hidden behind a barrier, doing her Overdive move to take control of a soldier on a raised platform. When selecting targets for Overdive, you can see in advance the weapons you'll gain access to once you've jumped over.
When actually blasting away at enemies, you have access to a lock-on button for locking on with a single button press. You can also switch off between weapons quickly, something that will be of importance given the variety you'll find in the Twisted foes.
The game also includes systems such as "Libertation" and "Radio" which will be detailed later.
You might have noticed that some CG scenes show Aya with her clothes torn. As you incur damage, Aya's clothes tear. This allows us to see more of the character, but it also means you'll take even more damage.
Famitsu notes that The 3rd Birthday's Aya has a slightly different image from the Parasite Eve Aya. There's apparently a reason for this that will be cleared up as part of the game's story.
