Masaaki Shirami

Masaaki Shirami (白身 正亜希, Shirami Masaaki) is a serial arsonist killed by Light Yagami as a part of an experiment to test the Death Note.

Plot
Light Yagami writes Shirami's name into the Death Note to test its effects. Light writes that Shirami will draw a pentagram inside a circle and then die.

At 6:00 PM on December 19, Shirami cuts his finger, draws a circle and a pentagram in his cell with his blood, and then dies.

Trivia
According to the manga, Shirami was kept in prison 21.