Eiichi Kite, universally known by his nickname Kiteretsu, is the main protagonist of the series. He is an elementary school student living in Tokyo, typically depicted as being ten or eleven years old and in the fourth or fifth grade. His defining characteristic is his extraordinary talent for invention and mechanical engineering, a genius-level intellect that sets him apart from his peers. Despite this high aptitude for creating gadgets, Kiteretsu has notable weaknesses; he struggles significantly with athletics and sports, and his academic performance in school is only average, as his focus is overwhelmingly directed toward his personal projects. He is often seen wearing his signature square glasses and a red or red-and-yellow sports visor featuring the letter K.

Kiteretsu's passion for invention is deeply rooted in his family history. He is a descendant of a renowned inventor from the Edo period named Kiteretsu Saisama, who is often described as a Japanese Da Vinci. The primary source of Kiteretsu's knowledge is an ancient manuscript known as the Kiteretsu Encyclopedia, which was created by his ancestor and contains designs for a vast array of miraculous gadgets. Much of his work is characterized by using simple, often pre-industrial materials to construct highly advanced devices, a concept sometimes referred to as bamboo technology. His inventive process is largely self-funded, relying on his allowance and New Year's money, which can lead to complications when his creations break down and he lacks the funds for repairs.

As a character, Kiteretsu is analytical, logical, and driven by an insatiable curiosity to explore new ideas and solve problems. He approaches challenges methodically, though his intense focus on his work can sometimes make him seem detached. Despite this, he genuinely cares for his friends and family. His primary motivation is to use his inventions to help people in trouble, although his creations are often impractical, prone to malfunction, and frequently lead to chaotic situations and unintended property damage. He is known to dislike dark places and has a fear of ghosts and other scary things, which contrasts with his otherwise rational and scientific mindset.

Kiteretsu's most significant creation and closest companion is a robot named Korosuke, whom he built using his ancestor's encyclopedia. Korosuke is a samurai-inspired robot who speaks with a distinctive verbal tic, ending his sentences with nari. He is often naive and lazy, serving as a comedic foil to Kiteretsu's genius. Their bond is incredibly strong and goes beyond that of a typical inventor and creation. His social circle also includes Miyoko Nonohana, a neighborhood girl who is his primary love interest. It is established through time-travel episodes that Miyoko is destined to become his future wife, and they eventually have a son named Eitaro. His other friends include Kaoru Kumada, a local bully nicknamed Butagorira (Buta Gorilla) who has a fondness for vegetables and later becomes a comic relief character, and Kouji Tongari, a wealthy but easily frightened boy who acts as a rival. An older university student named Benzo Karino also acts as a friend and mentor to the group.

Throughout the story, Kiteretsu experiences notable character development, growing from a boy solely obsessed with gadgets into a more responsible individual. This growth is particularly evident in the series' conclusions. In the manga finale, his mother disposes of the encyclopedias out of concern for his academic focus, leading him to give up inventing. The anime offers a different, more emotional resolution where the encyclopedias are stolen and lost, prompting a journey to the past to meet his ancestor. There, Kiteretsu learns that Korosuke was designed as a recreation of his ancestor's deceased son. In a mature act of selflessness, he agrees to leave Korosuke in the past to care for his ailing creator, demonstrating a profound sense of responsibility and love for his robot companion.

Kiteretsu's notable abilities are entirely centered on his inventive genius. He is capable of constructing a vast array of devices, many of which are listed in the encyclopedia. His most famous invention is a time machine, though it is generally restricted to traveling to the past rather than the future. Other well-known gadgets include a shrink ray or small light, which can change the size of objects and people; a magic umbrella; an underground vehicle called the Chitekyū or Subterráqueo; and various devices for aging, breathing underwater, or controlling animals. He is also shown to be capable of repairing normal machines and inventing his own original variations on the gadgets found in his ancestor's book.