Passive armor is an umbrella term encompassing occasions where a character experiences continuous armor without any restrictions. Examples of this include Tough Guy, Tough Body, and continuous super armor seen on transformation-related Final Smashes. Spirit abilities such as Super Slow Armor and Armor Knight can provide the effect to any character, albeit with statistical downsides such as decreased movement speed. Bosses can also be considered to have something akin to passive armor.
Passive armor was first formally introduced in Melee for Nana and Giga Bowser, where they have 5 and 20 units of subtractive knockback-based armor respectively. In Nana's case, the armor makes her slightly harder to launch than Popo, while for Giga Bowser it is intended to make him harder to launch, taking advantage of his massive weight value. As the armor is subtractive, attack that inflict knockback below the threshold won't launch either character, though in the case of Nana this is rarely seen outside of cases like Luigi's Super Jump Punch sourspot, because of the particularly low threshold and the Ice Climbers' below average weight; however crouch cancelling could slightly increase the effectiveness of the armor. Though Nana doesn't have any form of passive armor as of Brawl, the introduction of Bowser's Giga Bowser Final Smash introduced passive super armor for the first time. Smash 4 would expand on this further, adding Giga Mac, Mega Charizard X, and Mega Lucario, albeit with significantly nerfed durations to transformation Final Smashes in general. Smash 4 also introduced the Tough Guy mechanic for Bowser, giving him 19 units of knockback-based armor permanently with no drawbacks. With Giga Bowser no longer being a fully-controllable character in Ultimate, he received 95 units of knockback-based armor for his boss incarnation, rather than super armor. Ultimate's DLC would also introduce Tough Body as an effective clone of Bowser's fighter ability for Kazuya, only it uses 14 units of knockback-based armor.
Passive armor follows the general rules of armor — for example, grabs and some attack effects will override it — but this nature also leads to some unique side-effects, such as being present during the buried and frozen states, which can lead to delayed reactions to their effects. Passive armor is most commonly seen in the form of knockback-based armor as an extremely minor mechanic, such as the case with Nana or Kazuya, where the threshold is so low that it's effectively nonexistent in many matchups. Passive armor will sometimes come in the form of "subtractive" armor, wherein the knockback is subtracted instead of requiring a threshold to be reached to deal full knockback.
Ultimate introduced spirits, which can provide passive armor effects; specifically, Super Slow Armor and Super Armor from the likes of Regigigas, Polar Bear, Walhart, and Gold Mario can provide passive knockback-based armor to any character. The thresholds are almost identical, though Super Slow Armor, as the name implies, comes with a significant drawback in reducing the user's movement speed.
List of instances of passive armor
Tough Guy / Tough Body
Tough Guy and Tough Body are fighter abilities, originally introduced with Tough Guy in Smash 4. These specifically apply to Bowser and Kazuya, respectively, in the form of passive knockback-based armor, summing to 19 and 14 KB respectively. Giga Bowser also has the same mechanic, though it isn't named, and it has a threshold of 95 KB. This armor requires the armor threshold to be breached to deal hitstun and knockback. Nana and Giga Bowser both have abilities with similar effects in Melee, but it's subtractive, meaning that instead of requiring a threshold to be reached to deal full knockback, the knockback is subtracted by the value after calculation, preventing them to flinch if the value goes below 0.
Because Tough Guy and Tough Body's presence includes crouching, thanks to the 0.85× knockback reduction it provides, it improves the armor's ranges significantly; Kazuya in particular benefits from this as he will be crouching more often than most characters because of his unique crouch-based attacks. Bowser's armored smash and tilt attacks, as well as Kazuya's armored smash attacks and Heaven's Door, also benefit from the passive armor in Ultimate, giving him effectively two layers of armor for attacks to break through. Because the armor is knockback-based, Tough Guy's effectiveness deteriorates as percentage builds. As the character using it takes more damage, they take more knockback, which gives Tough Guy a hard limit in every matchup. Additionally, effects that increase knockback, such as rage, will also affect Tough Guy. For a character-specific example, Hero's Psyche Up applies a 1.2× knockback multiplier directly onto his moves which, as a result, significantly decreases Tough Guy's effectiveness—if not neuter it outright—until it goes away.
Due to Tough Guy's very low knockback threshold, its usefulness is highly specific to the point of being essentially nonexistent in some matchups. While characters like Luigi and Mega Man are noticeably impacted due to their reliance on weak, spammable projectiles in neutral, others such as Donkey Kong and Ganondorf are completely unaffected. This applies even moreso to Kazuya's Tough Body, having drastically lower damage ranges due to his lower base threshold and weight; for example, while Bowser can armor Pyra's Rapid Jab well until 174%, Kazuya can only armor it until around 12%. Like all armor, Tough Guy does not protect characters from grabs either, so characters that are noticeably affected by the ability can normally make up for it through a grab-heavy playstyle. Tough Guy was indirectly buffed in Ultimate thanks to the universal knockback reduction in all rapid jabs. The addition of damage-based armor into his moveset also assisted with this, adding a degree of safety at tighter percent ranges. Because Bowser himself gained more weight, it's common to see him armor rapid jabs to extremely high thresholds and punish them outright.
In the case of Giga Bowser, because he cannot be grabbed in any way and has gigantic weight, it is near-impossible to make him launch or even experience hitstun at first. Seeing Giga Bowser's Tough Guy break is extremely rare, but not impossible with charged smash attacks, strong items, or Final Smashes. However, whether the effort required to make him launch is a viable objective is very arguable.
List of attacks affected by Tough Guy/Tough Body