There are a number of Heartless that have continued to appear in multiple Kingdom Hearts titles after their first appearance, either in the original game or it's many sequels and spin-offs. However, while some enemies like Kingdom Hearts' shadows have stuck around throughout the series, the fan-favorite Invisibles has failed to appear in later titles.
These enemies have appeared in a handful of the later titles that retread over previous Kingdom Hearts games like Chain of Memories and Union X, but the Invisibles haven't been seen outside of these past references. Considering that Invisibles are a Pureblood Heartless as well, this makes for one of few omissions from later games as far as this specific pool of enemies goes.
When it comes to Pureblood Heartless, even bosses have reappeared in later games, most notably the Darkside boss that has returned in multiple different forms over the course of the series. The same has also happened with a handful of Kingdom Hearts' Emblem Heartless like Soldiers, which have also been given a few makeovers over the course of the series. In the case of the Invisibles, the most attention that these enemies have been given in the way of redesigns and being brought back into the series comes through some of the lesser appreciated titles.
Appearance in Underappreciated Titles
Many of the titles that Invisibles have appeared in, aside from the original, have been some of the fans' least favorite Kingdom Hearts games, including Re:Coded, 358/2 Days, and Union X. In the case of Re:Coded and Union X, the Invisibles appear as a part of the retreads over the original game, meaning that it's less that the Heartless has been brought back into a new game and more that its repeated in a recap. However, the 358/2 Days version of the enemy brought a new redesign called the Orcus that hadn't previously appeared during either the enemy's original outing or the Kingdom Hearts: Final Mix.
Fortunately, the series recap that is Kingdom Hearts: Melody of Memory list of songs also included the Invisible again, giving the Heartless a much better received title to appear in. This does also mean that there are now some newer models of the enemy that could be placed into whatever new title comes next in the series. So, maybe the fan favorite design of the original Invisible could be returning, as well as having 358/2 Days' Orcus come back in a meaningful way as well.
Design of the Invisible and Orcus
The designs of the Invisibles is what really makes them stand out among others, and most importantly the massive blue sword that the Heartless uses as a weapon. Interestingly, not many Heartless use weapons, with the few inclusions being Kingdom Hearts 2's Toy Soldiers, various humanoids like Bandits and Pirates, as well as a few sprinkled around some levels with various versions of guns. One detail on the Invisibles' weapons that makes this particular Heartless stand out, is the inclusion of a Kanji inscription that roughly translates into English as Hard Core.
Some fans have theorized that it might have been these over the top swords that eventually got the Heartless axed from future games, though that's somewhat unlikely with the newly designed enemies that appeared with weapons in future titles. In the end, while the design is one that is still beloved by fans and always a welcome inclusion in recap titles, the real issue is that the Invisibles likely just don't fit in many of the more Disney specific worlds, so there are few places to show them off. At the very least, 358/2 Days managed to get a variant of the Invisibles and the killer design still isn't forgotten every time Square Enix looks back at the series through titles like Kingdom Hearts Union X's ancient past storyline.
Kingdom Hearts: Melody of Memory is available now for PS4, Switch, and Xbox One.