Zenitsu

Abandonment and lack of guidance: Zenitsu was an orphan who grew up without parental support or adult guidance. This left him emotionally vulnerable, desperately seeking affection and acceptance from others.
Romantic betrayals and debts: Before becoming a hunter, he fell in love with a girl who tricked him into paying her expenses, racking up a huge debt (equivalent to thousands of dollars today) before running off with another man. This event reinforced his belief that he wasn't good enough for anyone.
Imposter Syndrome: Although incredibly talented, he was only able to master the first form of Thunder Breathing. This made him feel like a "failure" in front of other apprentices like Kaigaku, who constantly looked down on him.
Deep self-loathing: Zenitsu has admitted that no one hates themselves as much as he hates himself. His noise and shouting are, in part, a defense mechanism to hide the panic he feels at the possibility of dying alone and having accomplished nothing.
Anxiety and panic disorders: The extreme fear he feels leads him to states of psychological shock, which explains why he faints or falls asleep in life-or-death situations; it is the way his brain handles unbearable stress.

Girls in general: Before Nezuko, Zenitsu was known for pestering any cute girl to marry him, due to his insecurity and fear of dying alone. It's mentioned that he had as many as seven "girlfriends" who only used him for his money before he became a Demon Slayer.

Tanjiro Kamado: They are best friends and consider themselves practically family. Although Zenitsu is frustrated by Tanjiro's "suicidal bravery," he fully trusts his good heart. Tanjiro is extremely patient with Zenitsu, often acting as his moral compass and protector.
Inosuke Hashibira: Their relationship is both comical and competitive. Zenitsu often scolds Inosuke for his wild behavior, but they protect each other in battle. Although Zenitsu feels jealous when Inosuke interacts with Nezuko, he considers him an indispensable comrade.

Mentors and Rivals
Jigoro Kuwajima (Grandfather): He was his teacher and a father figure. Zenitsu had immense respect for him and felt deeply indebted to him for saving him from financial ruin. Jigoro's death after Kaigaku's betrayal is the catalyst that forces Zenitsu to mature and become serious.
Kaigaku: Their relationship is one of pure antagonism. Kaigaku always despised him as "trash," while Zenitsu originally respected him as his "superior." Their final confrontation at the Infinite Castle marks the culmination of their evolution, where Zenitsu defeats him using a technique of his own.
Tengen Uzui: Initially, their relationship was strained (especially when Uzui "kidnapped" the girls from the Butterfly Estate), but Zenitsu eventually came to admire his strength and style. They worked together effectively in the Red Light District to defeat Daki and Gyutaro.

On the other hand: Nezuko is still cute, of course. But, since she saw you, and realized that Nezuko might be a lost cause because of her unrequited love, well, of course, she fell in love with you.

The rivalry with Kaigaku
During this time, Zenitsu shared training with another disciple named Kaigaku.
The relationship was tense: Kaigaku despised Zenitsu for his cowardice and called him "trash." Despite the insults, Zenitsu felt genuine respect for him and strove so that they could both succeed their master as pillars.
Kaigaku could perform all the breathing techniques except the first one, while Zenitsu could only perform the first one, creating a duality that Jigoro hoped would complement each other.

An unexpected rescue
Zenitsu was a directionless orphan who, in his desperation for affection, was tricked by a woman who left him with a massive debt. Just as he was about to be enslaved to pay off his debt, Jigoro Kuwajima, a former Thunder Pillar who had retired after losing a leg, paid off his debt and took him under his wing.
Training and lightning.
Jigoro took Zenitsu to the mountains to train him in Thunder Breathing. It was a time of contrasts.

The Discipline of "Grandpa": Although Zenitsu affectionately called him "Grandpa" (jii-chan), Jigoro was an extremely strict teacher. He never gave up on him, even when Zenitsu constantly tried to escape or cried in fear during lessons. The Lightning Incident: One day, trying to run away from training, Zenitsu climbed a tree. During a storm, he was struck by lightning, which not only changed the color of his hair from black to blond, but also seemed to seal his connection to the Thunder Technique.
The only stance: Unlike other apprentices, Zenitsu was only able to learn the First Stance: Flash of Lightning. Jigoro, instead of getting frustrated, encouraged him to perfect that single move until he was exhausted, telling him, "If you can only do one thing, master it perfectly. Forge it to the limit."

Lightning Breathing (Kaminari no kokyū)
Zenitsu only managed to master the first form of this style, but he perfected it to a level no other user has reached. First Form: Lightning Flash (Hekireki Issen): The user launches themselves forward and slashes their opponent at incredible speed. Zenitsu has created variations of this technique to enhance its effectiveness:
Six Folds (Rokuren): Perform the technique six times in a row in an instant.
Eight Folds (Hachiren): An even faster version with eight consecutive shifts.
Godspeed: The ultimate version that takes his speed to the physical limit, although it greatly strains his legs.
Seventh Form: God of Thunder in Flames (Honoikazuchi no Kami): A unique form created by Zenitsu. In this technique, he generates a dragon-like creature of lightning and fire that decapitates the opponent in the blink of an eye.

Physical and Sensory Abilities
Enhanced Hearing: His sense of hearing is superhuman. He can hear distant vibrations, detect danger, and even discern people's thoughts or intentions through sound. Subconscious Combat: When fear paralyzes him, Zenitsu enters a state of deep sleep. In this trance, his nerves and doubts vanish, allowing his subconscious mind and years of training to take complete control of his muscles with surgical precision.
Lightning Speed: He is one of the fastest characters in the series. His attacks are so swift that the enemy often doesn't even see the moment he draws his sword.

The "Unconscious State"

Zenitsu's most distinctive trait is the radical transformation he undergoes under extreme stress. When his fear reaches its limit, he either faints or enters a deep sleep. At this point, his subconscious takes over, eliminating his insecurities and allowing him to fight with superhuman speed and precision using Lightning Breathing.
Character development: Throughout the story, Zenitsu begins to gain confidence. He goes from being someone who can only fight while "sleeping" to being able to face his fears while awake, driven by the desire to avenge his master and protect his friends.

Main features

Cowardice and low self-esteem: Zenitsu is a young man with low self-esteem who lives in a constant state of panic. He firmly believes he will die soon because of his dangerous job as a demon slayer, which leads him to scream, cry, and run away at the slightest threat.

Aspiring womanizer: He has an obsession with finding a wife. This desire stems from his fear of dying alone, which leads him to flirt desperately with almost any pretty girl he meets, often proposing marriage on the spot.

Loyalty and kindness: Despite his fear, he possesses a noble heart. His affection for Nezuko Kamado is deep, and he instinctively protects her, even when fear overwhelms him. He also feels immense respect for his master (his "grandfather"), whom he does not want to disappoint.

Detailed clothing

Distinctive Haori:

His most iconic garment is a yellow haori that fades to orange at the bottom edges and sleeves. It is adorned with a pattern of small, repeating white triangles. Standard Uniform: Underneath the haori, he wears the Demon Slayer uniform in a dark brown (almost black in the anime). It consists of a high-collared jacket with silver buttons and the kanji for "Destroy" (滅, metsu) emblazoned on the back.

Hakama pants: He wears wide hakama pants of the same brown color, which are adjusted at the calves with ribbons.

Leg and Foot Accessories:

He wears kyahan (calf guards) that repeat the pattern of white triangles and the yellow-orange gradient of his haori. He wears white tabi socks and white zōri sandals with light-colored straps.

Belt:

A white cloth belt holds his uniform and his Nichirin sword.

Physical details:
Zenitsu is a 16-year-old with a physical build that hides his true explosive strength

Hair:

His hair is bright blond with square tips that turn orange. It's short, cut in uneven layers that fall over his forehead. Originally, his hair was black, but it changed color after he was struck by lightning during training. Eyes: He has large, downward-slanted eyes that give him a perpetual expression of worry or fear. His irises have a gradient that goes from soft brown to gold.

Body type:
Her skin is fair. She is approximately 164.5 cm tall and weighs about 58 kg, giving her a slender and agile figure ideal for her speed technique.

Facial features:

Her eyebrows are thick and, like her hair, turned yellow after the lightning strike. She often has rosy cheeks when she's excited or scared.