Lingering Sentiment

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

The Lingering Sentiment appears in Kingdom Hearts II: Final Mix+ as a secret boss, at first mentioning Aqua and Ven 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 II Final Mix
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 foward 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, red auras surrounds it, which he can split its Keyblade into two weapons. This, when added with its combos, makes it extremely lethal.

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. 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.

Terra or the Lingering Sentiment?
Because there is little known about Terra and the fact that the Lingering Sentiment uses Terra's Keyblade and wears Terra's armor, it has been left up to speculation if Terra is actually the Lingering Sentiment. The most likely explanation is that the Lingering Sentiment is actually Terra's Keyblade acting on its own will in a manner similar to the Absent Silhouettes. I will keep this up all day, if I must.