Age: 16-17
Height: 168 cm (5'6")
Weight: 58 kg
Birthday: May 17, 1995

The main protagonist and the storyteller of the series. He is in his second year at Fumizuki Gakuen's High School and is an F-Class student.

Although he denies it, he is a rarely found 'fool' who even his own parents call him as "the fool over all the world." Not only are his grades the lowest, but he's really so stupid that he is titled the "Kansatsu Shobunsha (under observation)" from the school. For him his own safety is the most important thing and does not hesitate to use the most cowardly ways if needed, in circumstances where his harm is unnecessary.

However, he also has a righteous heart that can stand up against for other friends (except Yuuji). Even going as far as selflessly putting himself in harms way to help others, with no significant gain in sight for himself. Though shown he can have a perverted/active side, he's usually peaceful, gentle, kind and quiet towards those around him, which gains him certain popularity.

On the outside he is quite a bishounen, but is frequently thought (mostly by female students) to look better in a girl's outfit, and such photos and merchandise of him are secretly sold by Mutsurini—dubbed the hidden idol "Aki-chan."

He admires Mizuki but does not even dream of receiving love from her. While Minami has a heart for Yoshii, he thinks that he is hated by her after a series of events that almost killed him.

He entered Fumizuki Gakuen because his parents thought it was cheap. Yoshii's parents live overseas because of work, and for this reason he lives by himself. With nobody looking after him, he used up all his money for personal use and therefore was living in a poverty. His main food used to be salt and water—until his older sister returned back to Japan. Thanks to her supervision he couldn't spend money as he wished so his meals became normal once again.

However, because his sister and mother's housework abilities were zero, Yoshii is quite an expert when it comes to such areas. He is especially skilled in cooking.

He is right-handed, but he used to be a lefty (so says he).

Although his shoukanjuu's battle level is the lowest, he can comfortably stand up against high-class shoukanjuus thanks to his fluent ability to control it; however, because he is the Kansatsu Shobunsha, the fatigue and damage experienced by his shoukanjuu is returned to himself, he tries to avoid summoning during battles if not necessary.

Shoukanjuu's weapon is a modified Gakuran and a wooden sword. The Occult version is "Dullahan (headless knight)," influenced from the feature that "No head=fool." As told by Gakuen Principal, he is the one that is "distorting the essence of the Test Summon System."

As a side note, Yoshii placed 8th of the male characters and 13th overall in the 2009 edition of "Kono Light Novel ga Sugoi!"