If you're looking for manga similar to Hikari yo, you might like these titles.
This is the (imaginary) beginning story of the crazy popular Vocaloid Character GUMI!! By some freak incident, the now cyborg school girl has her eyes set on her idol debut as "Gumi" under the mysterious organization, "Intane Group"!?
The major power of the Evillious region, the Kingdom of Lucifenia. Ruling this vast country was the 14-year-old princess, Riliane. She used her absolute power to squeeze everything from the people, and anyone who defied her was beheaded without mercy.
What do you do when you have lost everything? Lloyd Lowell has found his answer. In Leon City, people are drawn to the torture of tower, Torcia Tower. There, it is rumored, lies treasures which can bring you fortune, fame and, most importantly—eternal life. However, nothing goes as planned when Lloyd enters the tower...
The story starts on April 21st, with him narrating the story. He was prisoner number 420; and briefly talked about the existence of "happy ends," negatively thinking about the world. Number 420 was then led to his cell by Kaimural (Kaito) where he found a blanket and walls of ceramic. Two new (non-vocaloid) characters were introduced, Rythm Garcha and Shazna Ivory; who tried to hurt number 420 to the point where he's shown to cry.