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 capabilities and limits of the Death Note.

Plot
To test the Death Note, Light uses six criminals in prison as test subjects. As written in the notebook, Yadanaka leaves a note before dying of a heart attack in his cell at 6:00 pm. 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. Though L deduces that these deaths might be part of an experiment by Kira and forbids the release of details of the deaths to the media, Light is able to access the test results through his father's login on their home network the same day.

The information he gains from the test results allows Light to successfully manipulate Kiichiro Osoreda in order to get the name of the man who has 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. It was written in the Death Note that he would die in front of the Eiffel Tower, but as it would be physically impossible to reach Paris in time, he inexplicably says the name "Eiffel Tower" to a prison guard before dying of a heart attack at 6:00 am.

L realizes that his death might be part of an experiment by Kira and requests the details not be made public, but information on his and two other criminals' deaths have already been reported to the media, and Light verifies his test results in the news the same day.

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 completes another successful test on Matsushiro Nakaokaji.