Annin is the guardian of the sacred Furnace of Eight Divisions, the border between life on Earth and the Other World. She is among one of the few beings working under King Yemma that does not appear to be an ogre, and instead looks to be somewhat human in physical form. Annin is immortal, and presumably ageless.

Biography
Annin is the sole pyre keeper of the Furnace of Eight Divisions, having secured the position for over ten thousand years. She is indeed very ancient due to this fact, even in spite of her attractive and youthful appearance.

Goku confronts Annin about the flames violently erupting on Fire Mountain, which had threatened to obliterate both the Ox-King and his entire village, to propose a hasty solution to extinguish the fires. Annin turns down this request for the sole intent of preventing demons from escaping Other World premises, as if the flames were to be extinguished even for a momentary period of time, they would be permitted total access to the world of the living.

Goku, of course, refuses to let up on his request and persists in attempting to convince Annin otherwise; this eventually leads into a brief bout between the two, in which Goku discovers her true age and unintentionally mocks her for it. Enraged at Goku's insolence, Annin transforms into a giant to pulverize the miniature-sized hero, employing various different methods in doing so (such as using her headpieces to attempt to stab Goku, as well as the gales caused by the flapping of her cape). Goku manages to counter her furious attacks due to the unrelenting power and force of the Bansho Fan, which consequently shocks Annin at its convenient and sudden presence.

Annin soon suggests an alternative method of extinguishing the flames, guiding Goku through the flames of the Furnace of Eight Divisions to quell the fire and thus, save the Ox-King and his village. Following her appearance, Annin is never seen nor mentioned again.

(Source: Dragon Ball Wiki)