Eitarō Oka attended junior high with Minato Kiyomizu, developing profound admiration for Minato's water polo prowess. He steadfastly supported Minato during his hospitalization and coma. Upon Minato's awakening, Eitarō actively pursued reconnection, transferring to Yamanami High specifically to follow him.

He relentlessly pushed Minato to revive his water polo career, urging him to rejoin the team with club captain Jō Jōjima. Eitarō initially presented himself as intimately familiar, fabricating a shared history and using nicknames to reinforce false closeness, driven by his personal investment in Minato's legacy. Discovering Minato's complete memory loss and vanished ability shattered Eitarō, causing visible distress and crushing disillusionment.

Confronted by Minato's need to rebuild from scratch rather than effortlessly excel, Eitarō briefly quit the water polo club. Minato later clarified his past success stemmed from hard work, not innate talent. This honesty prompted Eitarō's return, shifting his focus from idolization to genuine support for Minato's renewed efforts. Eitarō also aided club recruitment, leveraging Minato's past reputation to attract members like Yutaka Inomata and Yoshiharu Ushimado.

Eitarō possesses chestnut hair styled messily and wears black rectangular glasses over honey-yellow eyes. His expressionless demeanor often masks a headstrong, tenacious personality. His name (英太郎 - "excellent, thick/great son") combined with his surname Oka (岡 - "ridge/hill") hints at deeper narrative layers.