Yu Aida is a Japanese manga artist and illustrator born on November 8, 1977, in Tochigi Prefecture, Japan. He studied at Meiji University, where he was a member of the manga research club and began creating doujinshi, or self-published works. Following his graduation, he worked as an illustrator for a video game company, contributing character designs to the visual novel Bittersweet Fools.

Aida made his professional manga debut with the short story Flowers, published in Comic Mega Freak in 2000. His first major serialized work, Gunslinger Girl, began in Monthly Comic Dengeki Daioh in May 2002 and concluded in September 2012, spanning fifteen collected volumes. The series, which he both wrote and illustrated, is a seinen manga set in Italy and follows a secret government agency that turns young girls into cybernetic assassins. Gunslinger Girl received the Excellence Prize at the Japan Media Arts Festival and had sold over 2.5 million copies by 2007.

The manga was adapted into two anime television series. The first, produced by Madhouse, aired from 2003 to 2004. The second adaptation, titled Gunslinger Girl: Il Teatrino, was produced by Artland and aired in 2008, followed by two original video animation episodes. For Il Teatrino, Aida took on a significantly expanded role beyond being the original creator. He is credited for series composition, wrote the scripts for all episodes, provided overall supervision, and worked as a storyboard artist for the opening sequence.

After completing Gunslinger Girl, Aida launched the manga 1518! in Big Comic Spirits in August 2014. The series concluded in March 2019 and was collected in seven volumes. It follows a high school girl who becomes involved in student council activities centered on researching Japan's Sengoku period. His subsequent work, The Valiant Must Fall, began serialization in Hakusensha's Young Animal magazine in February 2021. Set during the transition from the Bakumatsu era to the early Meiji Restoration, the story follows a samurai navigating a rapidly modernizing Japan. The series remains ongoing.