Eir Aoi is a Japanese singer and songwriter known for her powerful, rock-inflected vocal style and a career deeply intertwined with the anime industry. Born on November 30, 1988, in Sapporo, Hokkaido, she grew up with a strong passion for music, playing guitar and serving as the lead vocalist for a band during high school. After graduating, she continued her musical pursuits by uploading cover songs to the video-sharing website Niconico, where her rendition of Piko's "Story" caught the attention of composer Fumio Yasuda. This discovery led to her major label debut in October 2011 with the single "Memoria," which served as the first ending theme for the television anime Fate/Zero. Early in her career, she adopted a mysterious "masked" persona, often covering her face in promotional materials, before revealing her full appearance during a live broadcast in June 2012.

Aoi's music is a signature blend of J-pop and rock, characterized by soaring melodies, dramatic string arrangements, and her distinctive, powerful vocals that often convey a sense of intensity and emotional depth. Her identity as an artist is closely tied to her collaborative work with other musicians and composers. For instance, her third single, "Innocence," which was used as the second opening theme for the 2012 anime Sword Art Online, was composed and arranged by Ryosuke Shigenaga, with lyrics co-written by Shigenaga and Aoi herself. The recording featured notable session musicians, including guitarist Masanori Mine and bassist Fire. This song became one of her most commercially successful tracks, earning a Gold certification from the Recording Industry Association of Japan for exceeding 100,000 digital downloads and charting on the Oricon Weekly Singles Chart for 28 weeks.

Throughout her career, Eir Aoi has provided numerous theme songs for popular anime series. Following "Memoria" and "Innocence," she contributed "AURORA" as an opening theme for Mobile Suit Gundam AGE and "Sirius" as the first opening for Kill la Kill. Her work on the Sword Art Online franchise has been extensive, including "IGNITE" for Sword Art Online II, "Iris" as the first ending theme for Sword Art Online: Alicization, and "Ryusei (Falling Star)" for Sword Art Online Alternative: Gun Gale Online. Additional notable credits include "Genesis" for Aldnoah.Zero, "Lapis Lazuli" for The Heroic Legend of Arslan, and "HELLO HELLO HELLO" for the anime series A Couple of Cuckoos. She also lent her voice to the video game Drakengard 3, performing its theme song "Kuroi Uta."

In October 2016, after several bouts of poor health, Aoi announced an indefinite hiatus from musical activities following two final concerts at the Nippon Budokan. She returned to the music scene in early 2018, signing with the SACRA MUSIC label. Her comeback was marked by the release of the single "Ryusei / Yakusoku" and a special live performance at the Nippon Budokan titled "RE BLUE." Following her return, she continued to release new music, including the album "FRAGMENT," the EP "Kodo" in collaboration with n-buna of Yorushika, and her fifth studio album "KALEIDOSCOPE" in early 2023. In January 2023, she announced another temporary hiatus due to health issues, and by the end of the year, she announced her departure from both her agency Guan Barl and her label SACRA MUSIC. She reactivated her career in 2024 as an independent artist.

Eir Aoi holds significant importance within the anime music industry, often being hailed as one of the leading "anime song divas" of her generation, with a passionate fanbase both in Japan and internationally. She has performed at major anime conventions and events across the globe, including Sakura-Con in the United States, AnimagiC in Germany, Japan Expo in Paris, and events throughout Asia. Her career, marked by both triumphant success and challenges with her health, has solidified her reputation as a resilient and immensely talented artist whose work has defined a substantial portion of modern anime soundtracks.