Daichi Manatsu is the main protagonist of the series, a seventeen-year-old high school student who becomes the pilot of the Earth Engine, a giant mecha created to defend the planet. He is defined by a proactive and enthusiastic attitude toward his role as a mecha pilot, which sets him apart from many reluctant heroes in the genre. Daichi is characterized by his strong sense of justice, straightforward nature, and a deep-seated desire to protect others. He is described as a generally nice person who is easy-going but becomes serious and determined when the situation demands it.

Prior to the events of the story, Daichi lived with his stepfather after his biological father, Taiyou Manatsu, died in a mysterious accident. His father was an astronaut, and his death is a lingering source of pain and motivation for Daichi, who later learns it was an act of heroism. His early life was also marked by a formative childhood encounter on Tanegashima Island, where he met a strange boy named Teppei Arashi on the other side of a fence at a secret facility. This childhood memory, along with the appearance of a mysterious rainbow circle in the sky, drives Daichi to return to the island at the start of the series.

Daichi’s primary motivation is to uncover the truth about his father's death and to combat the alien threat known as the Kill-T-Gang, who seek to drain the Earth of its life force. Unlike the stereotypical reluctant pilot, Daichi is excited and willing to take on the responsibility of piloting the Earth Engine, viewing it as a radical opportunity rather than a burden. His greatest plight eventually becomes the heavy responsibility and stress of being a captain, forced to carry the weight of the future for all of humanity. His role in the story is as the leader of a team of young pilots called the Midsummer Knights, assembled by the enigmatic defense organization Globe.

His key relationships are central to the plot. He shares a powerful bond with Teppei Arashi, a mysterious boy he befriended as a child who is later revealed to be a former member of the Kill-T-Gang with his memory wiped. Daichi is one of the few people Teppei trusts. Another crucial relationship is with Hana Mutou, a peculiar girl to whom Daichi is connected. He sees her for who she truly is and accepts her completely, culminating in a mutual romantic bond that he initiates by kissing her to show that his feelings are the same as hers. Daichi also works alongside Akari Yomatsuri, a skilled hacker who supports the team.

Throughout the series, Daichi shows a degree of character development. He starts as a somewhat generic but optimistic teenager grappling with his father's legacy, but he gradually grows into a more thoughtful and decisive leader. He evolves from being simply a "token nice guy" to a character who must confront the immense stress and moral complexities of his position, particularly in the latter episodes where his character is explored with more depth and solemnity.

Daichi possesses several notable abilities. As a Neoteny, he is able to summon and control a Livlaster, a gun-shaped artifact that generates and fires Orgone energy. He can summon this weapon to his hand from any location. The Livlaster has multiple functions, including a standard shooting mode that produces a powerful energy beam and a piercing mode that allows him to hit targets by penetrating their armor. This Livlaster is the key that allows him to pilot the Earth Engine mecha. Apart from his mecha-related skills, Daichi is also skilled with using boomerangs, though he is noted not to be a marksman with standard firearms. By the end of the series, he masters a teleportation ability called the Entangle Link, which he uses to travel from the planet Uranus back to Earth using only his Livlaster and the Earth Engine.