Shiniichiro Yamasaki

Shiniichiro Yamasaki (山前 芯市郎, Yamasaki Shin'ichirou) is a criminal killed by Light Yagami as a part of an experiment to test the Death Note.

Plot
To test the capabilities of the Death Note, Light Yagami uses six criminals in prison as test subjects, including Yamasaki. He attempts to make Yamasaki leave a message saying, "I know that L suspects the Japanese police," but this ends up failing due to the Death Note rules. Yamasaki doesn't know that L suspects the police, so therefore he could not write it.

Yamasaki dies of a heart attack at 6:00 P.M. on December 18th. The Task Force and L learn about it the next day, and the information is added to the NPA database. Light is able to access the test results through his father's computer the same day.

Light thought the test with Yamasaki might work, but he learned that the Death Note can't be used to make a person write something that they wouldn't know or think. This information, along with the other test results, allows Light to successfully manipulate Kiichiro Osoreda in order to get the name of the man who's been shadowing him.

Trivia

 * In addition to Yamasaki, Light's tests also fail on Yuzo Butsura and Hitoshi Kabeoka. Unlike Yamasaki, which Light thought might work, Light knew the other two tests would fail.
 * Light's tests work on three of the six criminals: Masaaki Shirami, Ken Yadanaka, and Tasayoshi Yoda. The next day, he does another successful test on Matsushiro Nakaokaji.