Lingering Sentiment

The Lingering Sentiment (留まりし思念) is the unlockable secret boss featured in Kingdom Hearts II Final Mix. It only appears after the player has completed every world and has defeated the final boss. After these conditions are met, a large portal opens in the Hall of the Cornerstone, in Disney Castle.

Kingdom Hearts II Final Mix
The Lingering Sentiment appears at first mentioning Aqua and Ventus and mistaking Sora as "the one he has chosen", then wonders if Sora is "Xehanort" before finally attacking him. Unlike Sephiroth, another secret boss in the game, the Lingering Sentiment fights against both Sora and his main companions, Donald and Goofy. After losing once, it can be challenged again, this time saying that it has nothing more to give to Sora except its hatred for "Xehanort". It's unclear why it actually attacks Sora. After the fight, it acknowledges Sora as a Keyblade Master.

Kingdom Hearts Birth by Sleep
It is revealed at the end of Kingdom Hearts Birth by Sleep that the Lingering Sentiment is, in fact, the armor of the Keyblade warrior Terra given life. After unlocking his heart, Master Xehanort proceeded to attack and possess Terra. The Keyblade wielder attempted to protect himself with his armor, this to no avail. Terra's armor shattered to the ground, Xehanort born. As Master Xehanort moved forward in his new body, he found himself locked in an energy field. Behind him stood the Lingering Sentiment, a battle ensuing. After the battle, Xehanort lay unconscious, disappearing in a stream of light. The Lingering Sentiment kneeled down, vanishing from view as Kingdom Hearts disappeared above.

Journal Entry
A man connected to the portal﻿ which appeared at Disney Castle.

The armor covering his body can master the Keyblade´s power much more than Sora.

Strategy
It, like the game's other secret bosses, is difficult to defeat; its most basic moves are attacking with its Keyblade (using a combo attack almost identical to Sora's, except that at the end it turns its Keyblade into a drill-like object for the forward thrust), and can fly at very high speeds. Its main abilities are transforming its Keyblade into various attacking forms. It can turn it into a whip that does less damage than the blade form, but its greater speed and reach makes up for it (it's so fast that if Sora is hit by it he won't be able to dodge or block the other hits), or a massive cannon (which is even bigger then the Lingering Sentiment itself) that fires out a large, very powerful ball of energy (Sora, however, can deflect this back to the Lingering Sentiment, which leaves it very vulnerable), a jet-ski like object that lets it fly extremely fast, a cog-like object that spins around so fast that if Sora gets hit by it once, then the rest of the hits will also hit Sora. It can also conjure a "ritual" like attack, which undergoes into a berserker-like condition (similar to Saïx's Berserk Technique) in this form, its arm gauntlets and helmet will emit a bright red, which he can split its Keyblade into two weapons. This, when added with its combos, makes it extremely lethal. It's only vulnerable after completing an attack or after deflecting the cannon shot back at him (as mentioned above). Any other time that you attack him will do nothing at all.

In addition to its Keyblade, the Lingering Sentiment has other abilities as well. It can also create an unusual looking seal that either creates an orange or blue orb. An orange orb causes Sora to be unable to attack and a blue orb prevents Sora from using magic or items. It can also conjure drone-like objects which fire plasma at Sora and his party. A particularly deadly attack is trapping Sora in a triangle of objects that drain his heath at a very fast rate (similar to an attack used by Xemnas, but much more powerful), and only a "Break Free" command on Sora command list can save him from it. If the player chooses the wrong command while he is trapped, then Sora will immediately die (even with Once More and Second Chance equipped). Interestingly, when it strikes Sora with its Keyblade(s), most of its combos resembles Sora's combos, most notably the Valor and Final Form combos.

The Lingering Sentiment is the most powerful enemy in Final Mix+ and is arguably the hardest boss in the series. Its level of power compared to all other bosses in the series may be a foreshadowing of sorts to something as-of-yet unrevealed.