Vanitas (HFAOW)

Vanitas is (in regular canon) Master Xehanort's apprentice, Ventus's dark half, the creator of the Unversed, and a Keyblade Apprentice all-in-one

In regular canon, after helping his master submit Terra to darkness, as well as shatter Ventus's heart and shake Aqua's will, he was finally killed in the Awakening by Ventus while Aqua held off Ventus's possessed body alongside King Mickey.

Heroes From An Old World
In HFAOW, Vanitas first appeared after Scaith gave the Vanitas Sentiment back his sentinence at the Keyblade Graveyard. Shortly afterwards, Vanitas fought and defeated the Lingering Sentiment in a short amount of time. Deciding to call Scaith his "Master" for now, Vanitas followed Scaith and his messenger imp Lily back to Scaith's dark city (formerly the Sonic OVA's Robottropolis in the Land of Darkness).

Vanitas, for the next couple of scenes he is shown in, is shown sending Flood Unversed around the continent of Belrain to scout the area and figure out who lives where and why. Soon enough, Vanitas is ordered to find and kill Warp the Hedgehog, Scott McCarthy The Fox, E-1,000 Theta, Gamble the Echinda, and Furry the Fox. Obeying orders, Vanitas donned his helmet and snuck off to the ice fields, where Warp was already underway in his quest to save his girlfriend Nebula from Dr. Madness; at the same time, Aero was making his way to the ice fields, and Scott was fighting off Scratch, Grounder, and Coconouts in their M6 mech. After Scott won the fight and Aero reunited with him, Vanitas appeared to them, summoned his Keyblade, and battled the two.

Vanitas made quick work of Scott, doing an afterimage attack after supposedly being knocked out (which is remniscient of his loss to Aqua at Radiant Garden). This act of violence provoked Aero to turn into his powerful Darkspine Aero form, which proved to be an equal match to Vanitas. Vanitas tried to steal Scott's heart to turn him into an Unversed, but this plan failed due to Scott still having the will to go on. Enraged, Vanitas summoned a Trinity Armor to deal with Darkspine Aero while he dragged Scott to the Realm of Darkness.

Once inside the Realm of Darkness, Vanitas revealed his plans to the fox about hoping to re-obtain the X-Blade, but before he could kill Scott, Aqua showed up, healed Scott, and engaged in battle with the dark enigma. Vanitas appeared to be defeated, but--in his pride--he summoned six Darkside Heartless to finish off the two. In a scene nearly mirroring Aqua's ending in Kingdom Hearts: Birth By Sleep, Terra's and Ventus's Keyblades shine through the Realm of Darkness and kill all six Darksides before knocking Vanitas to the ground. Scott delivered a powerful kick to Vanitas's head, shattering part of his helmet. Vanitas ran off before he could be identified, leaving Aqua and Scott stranded.

He soon reappeared back out in the ice fields, having used the power of darkness to mend his helmet. Looking down off an icy cliff, he saw Warp being strangled by E-10,000 Ultranos, while Nebula was trying to fend off the powerful robot. Gamble and Furry were apparently trying to escape, but Vanitas charged downwards and stuck his Keyblade in the ice, shattering the entire ice field before anyone could move. This act sent the group down into an underground icy cavern with no escape routes, leading Vanitas to fight the five. Vanitas eliminated E-10,000 Ultranos quickly, although (unknown to anyone) he was still active and was observing and copying Vanitas's data and attacks. Vanitas soon went onwards to heavily injure Warp, as well as make small enigmatic quotes about Nebula's Super Form.

Vanitas soon was beaten by Scott and Aqua appearing in a flash of light, with Aqua attacking Vanitas and knocking him to the ground, similar--if not the same--to what Vanitas did to Aqua before his penultimate fight with Ventus at the Keyblade Graveyard. Scott--now sporting the Tornadosword (which wasn't a Keyblade, due to its lack of summoning ability and no Keychain)--helped his friends to his feet to stop Vanitas.

Vanitas, however, summoned a total of 1,000 Unversed: all of them comprised of Floods, Scrappers, Arch Ravens, Buckle Bruisers, Thorn Bites, and Wild Bruisers. Vanitas fled the scene while ordering his minions to kill everyone that wasn't an Unversed.

Vanitas later found out about Scaith summoning Xaroe from Aero's body, making a second Unversed with the potenial to wield the X-Blade. Vanitas, angered by this, threatens Scaith, but Scaith warns Vanitas about his prideful nature, to which Vanitas silently leaves the room.

Vanitas later appears to Warp when Nebula is dying, saying her death could've been prevented if Warp had just been faster and smarter. Vanitas then proceeds to call Warp both Beelzebub and Satan, before leaving him after poisoning his heart with darkness (unknown to Warp, but the reader can tell, since Warp had been shown with a dark black aura around him during his small bits of speech during Vanitas's monolouge). Vanitas's (current) final appearance was to Scaith, saying he successfully poisioned Warp with darkness, and he was then ordered to either kill Nebula or turn her into an Unversed.