Tenacity

With Tenacity equipped, the Special Gauge automatically fills anytime the wearer's team has more inactive (waiting to respawn) players than the enemy team. The Gauge fills faster the greater the disparity between the teams. This ability is restricted to headgear as a main ability, and cannot be rolled as a sub ability.

In-Depth Ability
As of 2.6.0, the total time it takes to fill the Special Gauge without covering ground with ink:

Use
Tenacity is triggered if there are fewer active teammates than the opposing team. This will gradually increase the player's Special Gauge until their team's number of active teammates is equal or greater to that of the enemy team. The rate at which the Gauge is filled is dependent on how many teammates are inactive. This ability is an ideal when being used in Private Battles when there are less than four Inklings in your team.

As of Version 2.6.0, Tenacity also has a Special Charge Up effect when active, with the special gauge filling 20% faster than normal.

Gear
Tenacity is the main ability of:

Mutual exclusivity
Since this ability is found as the main ability only on headgear, other abilities found only on headgear are mutually exclusive with it; in other words you cannot have them together. These abilities are Last-Ditch Effort, Opening Gambit, and Comeback.