En is depicted with dirty blond hair that reaches the base of his collar, with his bangs parted to the sides and a single strand falling across his forehead. He has blue eyes and is among the tallest members of the Earth Defense Club, giving him a more mature presence compared to his teammates.

His Binan High School uniform consists of long black trousers, a white shirt left untucked, and a black blazer worn unbuttoned. The blazer is accented with a red pocket square, and he completes the look with brown shoes, reflecting his relaxed and informal attitude.

When transformed into the Flashing Prince, Battle Lover Cerulean, En wears a regal, prince-inspired costume. The outfit includes a white jacket with puffed sleeves and blue upturned cuffs, extending into six long coat tails lined with blue fabric. A blue bow with a winged, heart-shaped jewel rests at his collar, and a frilly undershirt peeks out beneath the jacket. He also wears white silk gloves, form-fitting white pants, and blue boots trimmed with white at the sole and heel. The design emphasizes his water element, with cool tones highlighting his association with calmness and fluidity.

En is characterized by his relaxed and easygoing nature, serving as a foil to the serious and disciplined personality of Atsushi Kinugawa. He often approaches situations with a laid-back attitude, appearing unmotivated or indifferent. However, when circumstances require his full attention, En demonstrates sharp focus, quick thinking, and even leader-like qualities, stepping up to guide or support his teammates.

He is rarely prone to anger and tends to be accepting of most situations, contributing to his image as the calmest member of the group. Despite this, Episode 8 reveals that En has unconventional and extreme methods of venting frustration or sorrow, hinting at deeper emotional complexity beneath his outwardly relaxed behavior.

En Yufuin is introduced as a third-year student at Binan High School and one of the five founding members of the Earth Defense Club. Unlike many of his peers, En prefers to live at his own pace, often skipping unnecessary exertion in favor of relaxation. Despite his tendency toward idleness, he becomes an integral part of the Defense Club after encountering the mysterious pink wombat-like creature that bestows the ability to transform into a Battle Lover.

As Battle Lover Cerulean, En is assigned the element of water, a role that aligns with his calm yet occasionally unpredictable personality. While he initially shows little interest in the responsibility of defending Earth, he grows into the role, proving himself both resourceful and dependable in battle. His bond with fellow club members, particularly Atsushi Kinugawa, helps anchor him to the team, balancing his disinterest with loyalty and occasional bursts of initiative when his friends need him most.

Atsushi Kinugawa
En shares his closest bond with Atsushi Kinugawa, his fellow third-year student and teammate in the Earth Defense Club. Though their personalities are nearly opposite Atsushi being serious and disciplined, while En is laid-back and carefree their contrasting traits complement each other. They are often seen together, and their friendship is highlighted by the casual nicknames they use for one another, with Atsushi calling him “En-chan.” Atsushi frequently encourages En to be more proactive, while En helps Atsushi relax and avoid overexertion.

Yumoto Hakone
En initially finds Yumoto’s boundless energy and innocence unusual, yet he quickly grows fond of him. He shows a protective side toward Yumoto, particularly during moments of crisis, such as when Yumoto’s brother was kidnapped. Despite finding Yumoto’s eccentric behavior baffling at times, En respects him as both a friend and a teammate.

Earth Defense Club Members
En maintains a strong sense of camaraderie with the other members of the Defense Club, Io Naruko and Ryuu Zaou. While he is not as openly expressive as Yumoto or Atsushi, he values their companionship and contributes to the team dynamic with his balanced, level-headed presence.

The given name En (煙) translates to “smoke” or “fume,” reflecting a natural element often associated with obscurity or transience. His surname, Yufuin (由布院), combines the kanji yu (由, “reason” or “cause”), fu (布, “cloth”), and in (院, “institution” or “hospital”).

The surname is also derived from Yufuin, a well-known hot spring resort in Ōita Prefecture, Japan, located approximately ten kilometers inland from the larger hot spring destination of Beppu. This ties into the broader theme of the Binan Koukou Chikyuu Bouei-bu LOVE! series, where many characters’ names reference famous hot spring locations.