Ventus-Vanitas is the final boss in Aqua's story in Kingdom Hearts Birth by Sleep. He wields the χ-blade as a result of Ventus and Vanitas's fusion, with Vanitas controlling Ventus's body in battle against Aqua.
Ventus-Vanitas, as his name suggests, is the merged form of Ventus and Vanitas. The boss wears Ventus's Keyblade Armor upon his creation, but after Vanitas reveals himself to be in control of Ventus's body, the antagonist's black and red bodysuit replaces it. Ventus-Vanitas is physically identical to Ventus, but he has Vanitas's gold eyes and uses Vanitas's battle stance.
Ventus-Vanitas's name is a combination of Ventus and Vanitas's names, which mean "wind" and "emptiness" in Latin, respectively.
At the beginning of this climatic confrontation, Lock-on to Ventus-Vanitas and press when prompted to execute the Burst of Faith command with Mickey. This command grants Aqua much-needed invulnerability, deals continuous damage so long as is tapped repeatedly, and possesses a powerful Finisher that will send the boss flying and interrupt any attack he attempts. Use Seeker Mine to interrupt Ventus-Vanitas as he rushes around the battlefield in a Sonic Blade-like fashion, and if the boss ever dives underground and becomes visible only through a moving pool of darkness, chain multiple Cartwheels together to avoid the barrage of fireballs that erupt as he attacks Aqua from below. Cartwheel can be used to avoid the crescent-shaped wave of energy Ventus-Vanitas launches after his two-hit χ-blade combo, and it is also an effective way of dodging the wave of dark energy bolts he sends towards Aqua from above. If Ventus-Vanitas is knocked off his feet at any time, he drops down from above and strikes Aqua with a χ-blade swing that spawns large icicles when he lands. Cartwheel is the best means of avoiding damage.
When Ventus-Vanitas's HP drops to about half of its total amount, he gains access to an ultimate attack whose execution is signified whenever the boss warps to the center of the battlefield and begins charging power. It is at this time that Aqua should be the most aggressive towards him, but be sure to execute Burst of Faith or a Shotlock as soon as he leaps into the air to avoid the intense burst of energy that follows. There is a small chance that Aqua can avoid this attack by repeatedly using Cartwheel along the very edge of the battlefield, but it is important to use Curaga immediately afterwards if she is hit. Depending on Aqua's level and stats, this energy burst can very well spell her end.
In terms of countering Ventus-Vanitas, the boss is best attacked with commands skilled at following mobile targets, such as Firaga Burst and Time Splicer. Even Shotlocks are useful. Ventus-Vanitas is slightly more intelligent than Vanitas is when it comes to avoiding Seeker Mine, so Aqua should be on her guard at all times. Ventus-Vanitas is swift, and he attacks almost unceasingly. Therefore, a continuous hit-and-run strategy should be used if one wishes to survive this fight. Aqua need not worry about Mickey's HP if it is fully depleted, as he revives himself in a matter of seconds. His attacks, in combination with Aqua's, should easily be able to conquer Ventus-Vanitas.