Amesu, also known as Ameth, began existence as the navigation fairy Fio. Serving as an NPC Guide Fairy in Legends of Astrum, she gained sentience and became the personal guide for Yuuki, accompanying him and the Twinkle Wish guild. She provided critical knowledge about monsters, locations, and items while exhibiting a friendly demeanor with an excitable love for adventure. Her personality included dry humor and a habit of bluntly pointing out allies' mistakes, which annoyed Yuuki and Rei. She keenly observed interpersonal dynamics like Yui's crush on Yuuki and occasionally revealed game secrets to aid against threats like the Seven Crowns.

During the Tower of Sol conflict against Kaiser Insight, Fio collaborated with Yui to free administrator Minerva from Mana Senri's control. This act made Fio a deletion target. To survive, she merged with data from Ameth—a bug-ridden prototype Guide Fairy and "ancestor" to all such entities. This fusion enhanced her capabilities but caused instability. Adopting the name Ameth, she embraced a new identity symbolizing "truth," derived from ancient terminology and the Sigillum Dei diagram.

In Re:Dive, Ameth resides in a separate domain, rectifying world abnormalities while recovering from near-deletion. Her appearance transformed: once a fairy with green hair, light-green wings, and white-and-gold attire, she now sports purple-ombre hair, blue eyes, and mechanical wings resembling damaged machinery. Her black clothing features purple diamond patterns, with tattered hair ornaments and skirt. Personality-wise, she became jaded and withdrawn, losing her former exuberance but retaining blunt, dry humor. Despite detachment, she cares for Yuuki and Twinkle Wish—personally tending to amnesiac Yuuki and monitoring their well-being. She intentionally withholds painful truths to protect Yuuki's psyche, though implying a future reunion.

Ameth occasionally interacts with past allies. In battles, unique dialogues trigger with Hiyori, Yui, Rei, or Kokkoro; the first three address her as "Fio." Her Union Burst briefly reverts to Fio's voice if Hiyori is present. Functionally a non-combatant healer and omni-support unit, she uses skills like "Tore Splendente" to boost allies' physical/magic attack, provide damage barriers, and recover TP. This skill's effectiveness scales with her magic attack and requires allies' active damage barriers for full benefits.