666 Satan (O-Parts Hunter) is one of the best examples of the "Hidden Power" trope from the early 2000s. Since you've been looking into high-stakes series like Academy's Genius Swordsman and 4 Cut Hero, this manga offers a similar thrill but with a finished, 19-volume conclusion.

Here is a breakdown of the Power System and World Stakes that make this a classic:

The O-Parts System (Ancient Tech)
In this world, "Magic" doesn't really exist—instead, people use O-Parts. These are relics from a lost civilization that only "Tacticians" can activate with their spirit (Mana).

Rankings: O-Parts are ranked from E to SS.

Effect: They don't just grant power; they transform. One O-Part might be a simple boomerang that doubles in size, while another could be a massive satellite that fires lasers from space.

The "Double" Tactician: Jio Freed is rare because he can synchronize with multiple O-Parts, allowing him to combo abilities in a way that modern "System" protagonists often do.