Ken Yadanaka

Ken Yadanaka (矢田中剣, Yadanaka Ken) 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 uses six criminals in prison as test subjects. As written in the Death Note, Yadanaka leaves a note before dying of a heart attack in his cell at 6:00pm. The note contains part of a coded message for L.

The Task Force and L learn about the deaths the next day, and the information is added into the NPA database which is accessible through Soichiro Yagami's account. Light is able to access the test results through his father's login the same day.

The information he gains from his test results allows Light to successfully manipulate Kiichiro Osoreda in order to get the name of the man who's been shadowing him.

In other media
In the 2015 Death Note television drama, his name is written as "Yatanaka" in the official subtitles. He is used by Light to test the limits of the Death Note, but the circumstances of his death are different from canon.

Yatanaka is a death row inmate in the Chiba Prefecture. At 6am, he inexplicably says "Eiffel Tower" to a prison guard before dying of a heart attack.

Trivia

 * In addition to Yadanaka, Light's tests also work on Masaaki Shirami and Tasayoshi Yoda. His tests fail on Shiniichiro Yamasaki, Yuzo Butsura, and Hitoshi Kabeoka. The next day, he does another successful test on Matsushiro Nakaokaji.