[4] A graduate from the Visual Concept Planning Department of Osaka University of Arts, Koizumi studied film, drama, animation and, to a lesser extent, storyboarding.

[4][5] He had originally intended to become a film director but applied at Nintendo instead to pursue his goal of creating a kind of drama only experienced in video games.

[6][9] While developing Super Mario 64, Nintendo's employees devised rough concepts of a three-dimensional The Legend of Zelda game with a bigger focus on puzzles and less pronounced action elements.

When he joined Toru Osawa and Jin Ikeda, he was the third staff member to work on the game that would become The Legend of Zelda: Ocarina of Time.

Koizumi observed how only one of the ninjas would attack the main samurai at a time while the others remained in a waiting pattern, which proved to be the solution to designing battles with multiple opponents.

[11] Koizumi replaced the simple triangle the team had implemented to mark the player's focus with a fairy that would change colors based on the friendly or hostile nature of the Z target.

[16] Koizumi carried over his "cops and robbers" idea with the time limit and expanded it into a world threatened by a falling moon after he had daydreamed about such a scenario.

Koizumi directed Super Mario Galaxy and was heavily involved in the creation of its story, deciding on the inclusion of the optional fairytale book that tells the characters' backstory.

When creating fictional worlds, he tries to surprise players with visually impressive environments and difficult gameplay mechanics while still ensuring "ease of use", that is effortless navigation and control.

He said that most of his time on a video game is spent working on player characters and their abilities in order to strike a balance between "fun and complexity" in control.

Koizumi mentioned tempo and rhythm as some of the most important elements of a Mario game, whereas he gave anticipation of the awaiting challenges as a crucial factor in a Zelda title.