Ventus-Vanitas

Ventus-Vanitas is the final boss in Aqua's scenario in Kingdom Hearts Birth by Sleep. He wields the completed χ-blade as a result of Vanitas and Ventus's fusion, and forces Aqua to battle her possessed friend in the physical world while Ventus struggles to defeat Vanitas from within his heart. Vanitas is in control of the fusion, and so the gestalt acts just as Vanitas would have done.

Design
Ventus-Vanitas, as his name suggests, is the merged form of Ventus and Vanitas, and combines characteristics of both characters. He usually wears Vanitas's black and red bodysuit from the neck down, though he lacks the helmet. Immediately after forming, he was still wearing Ventus's normal clothing, but quickly donned the bodysuit. Physically, Ventus-Vanitas is nearly identical to Ventus, but he has Vanitas's golden eyes. His battle stance is identical to Vanitas's.

Ventus-Vanitas's name is merely a combination of Ventus and Vanitas's names, which mean "wind" and "emptiness" in Latin, respectively.

Strategy
As Ventus proceeds to battle Vanitas inside their Awakening in his heart, in the physical realm, Aqua, having been ambushed by Vanitas after her battle with Braig, but having also been rescued by King Mickey, finds Ventus's body under the possession of Vanitas, and is forced to fight him in order to stop Vanitas from opening Kingdom Hearts with the completed χ-blade and to free Ventus. King Mickey will assist Aqua in this battle, and she also has access to Shotlocks, and Command Styles.

Ventus-Vanitas will immediately go on the offensive, and it is best that the player keep at a constant hit-and-run strategy to escape him. It is also important to remember the fact that while Aqua is very agile, her physical attack techniques aren't very powerful, and therefore her superior magic abilities must be relied on. For this reason, Mine Square can be used to interrupt the boss's attacks and deal great damage. Other spells will deal great damage, as well. It is also important that Aqua utilize Mickey's presence to her advantage, as not only are his attacks powerful and his HP in no need of restoration, but he grants her the use of the Burst of Faith ability. This attack makes Aqua immune to damage during its execution, having a long range as well.

The boss may occasionally sink into the ground, visible only by a pool of darkness. This pool will go to the player's location, and Cartwheel must be used to escape the rain of fireballs that Ventus-Vanitas summons as he strikes from below. Note that he can perform this attack multiple times. He may also attempt a physical combo, one that can be guarded against with Barrier and used to the player's advantage by opening a window for the boss to receive damage with Counter Blast. If the boss vanishes, immediately Cartwheel away before he hits Aqua with a downward slam with an added Blizzard effect. While Ventus-Vanitas has access to several other devastating physical desperation attacks, such as chaining physical combos with bursts of energy and a Sonic Blade-like attack, his most devastating by far causes him to warp to the center of the battlefield, charging power. He will then leap into the air, slamming back down in an immense burst of light. You are able to dodge this by continuously cartwheeling around the edge of the battlefield. Other ways of dodging this is by using the Burst of Faith Reaction Command or use a finisher that will last long enough. Failure to do so may spell certain death depending on Aqua's HP.

As long as the player remembers that Aqua favors magic over strength and uses magic-based abilities to the fullest, Ventus-Vanitas should fall relatively easily. Heal when necessary, make full use of special powers and abilities granted by King Mickey or Command Styles, and maintain a hit-and-run strategy.

Trivia

 * Ventus-Vanitas shares similarities with Riku while he is possessed by "Ansem" in Kingdom Hearts; both wield an incomplete Keyblade formed from pure hearts meant to unlock Kingdom Hearts, have a similar outfit, and speak with a dual voice, although Ansem's voice becomes more distinct over time. Furthermore, another character makes a similar comment on the gestalt "not being" their original self (Sora tells the possessed Riku "You're not Riku!", while Mickey tells Aqua "That's not Ven!")