En Yufuin is a third-year Binan High School student and founding member of the Earth Defense Club. His personality is overwhelmingly laid-back and self-paced, frequently perceived as lazy or slothful. He prioritizes relaxation, favoring naps and visits to hot springs, and wears his school uniform untidily, often without a tie. Despite this languid exterior, En reveals sharp vigilance and logical thinking when action is required, sometimes stepping into leadership during critical team moments. He harbors sensitivity about his mature appearance making him seem older, which troubles him. His emotional vulnerability manifests intensely, particularly in anger or sorrow triggered by conflicts threatening his closest bonds. He exhibits a pronounced panic reaction to cockroaches.

A deep, long-standing friendship with Atsushi Kinugawa defines his background, originating from a chance middle school encounter near a curry restaurant. This bond is described as essential as "air" or "oxygen" to En's existence. Their dynamic involves mutual reliance, with Atsushi frequently aiding En in practical matters like waking him or preparing meals. This closeness inadvertently caused jealousy and a rift between Atsushi and his childhood friend, Kinshiro Kusatsu. En's family name originates from the Yufuin hot spring resort in Ōita Prefecture, reflecting his fondness for onsen culture and snacks like onsen manju. He enjoys seafood curry but dislikes chikuwa-bu.

As a Battle Lover, En transforms into Battle Lover Cerulean using the phrase "Love Making!" and a bracelet kiss. Representing water with a blue theme, his attacks include the pressurized water stream Cerulean Aqua and the upgraded Cerulean Love Aqua in later seasons. He wields the Purifying Aqua love stick, crowned by a milk-shaped emblem. Viewing combat as bothersome, he prefers swift resolutions to return to relaxation. His abilities encompass combination techniques like Love Attack and Love Shower with fellow Battle Lovers. In battle, he rescues teammates when needed, such as aiding Yumoto Hakone during a transformation failure. His tactical ingenuity surfaces in non-combat scenarios, like winning an arm-wrestling match through clever distraction.

Across various media, En's relationship with Atsushi evolves with nuanced emotional layers. Official sources, including light novels and an otome game, depict their bond with romantic undertones. In the game, En explicitly confesses developing same-sex romantic feelings for Atsushi, seeking advice on managing them. This complexity surfaces during external manipulations; for example, a monster-induced conflict leads En to nearly abandon the team after clashing with Atsushi, only to reconcile by heroically rescuing him. In the OVA, he participates in group affection without reservation when not influenced by external factors like drugged chocolates. Season 2 amplifies his abilities with enhanced transformations and attacks, aligning with broader team upgrades. Spin-offs like "HAPPY KISS!" reference his legacy indirectly through new characters inheriting similar thematic roles, though En himself does not appear.

He engages in club activities with a preference for traditional entertainment, such as enka singing programs during New Year's events. His trivia-sharing habit earns him the nickname "trivia king" among peers. Despite his aversion to effort, he achieves high test scores through educated guessing and contributes strategically during school contests, highlighting underlying competence beneath his languid exterior.