Endurance

An Emitter Quirk listed at activation range, Endurance is the power Gigantomachia held before he ever carried any others. It surfaces in Chapter 273 and Episode 118, with volume omake material confirming it as his native Quirk.

The Quirk converts motion and morale alike into raw bodily energy, pouring the result back as stamina and staying power. Volume notes sum the outcome up as a stamina beast. The effect scales with feeling: the more excited Gigantomachia grows, or the more fixated he is on finishing a job, the more force and vigor he has to spend.

The durability it grants runs deeper than combat. A body this resilient can hold and use several Quirks at once with no penalty to body or mind, and without the surgical remodelling the Nomu require. That property is the whole argument for his usefulness as a bodyguard, since his unending devotion to whoever he serves, All For One first and Tomura Shigaraki later, supplies the motivation feeding the Quirk in the first place.

Sustained punishment with nothing to renew his spirit will eventually drain him. The loyalty requirement cuts deeper still. Waiting on a command leaves him inactive and unable to help, which is why he failed to respond when the Gunga Mountain Villa first came under attack, and he grows markedly weaker until ordered, as when Best Jeanist's binds held him better than they otherwise might, though a sedative was taking hold at the same moment.

Gigantomachia is the only user. Endurance is the foundation beneath the rest of his arsenal, letting him stack additional Quirks a normal frame could not tolerate, and it converts his fanatical service into raw physical output.