Plot had some kind of potential, training for millions of years to become more powerful is an interesting concept. That concept could have involved a lot of character development and traits that our main protagonist would have. As who would not loose themselves during that period of time? The character motivation factor is really weak and poor to keep himself up and the time period of the training was very short. It is a perfect representation of a someone failing at something and failing it forever barely gaining anything great except thanks to plot. If you spent training for that much time and still are mediocre afterwards inthe academy standard. My question would be how did he waste his time learning nothing.

The button barely comes into play again further wise so it is purely for plot convenience this macguffin was added. At lot of the psychological impact has been ignored and is barely touched upon.

It is a good bus trip read or when exceptionally bored. The art is good nothing impressive.
This story about the button should just be like a special button allowing you to train for 1month or 1year as the character is not overwhelming powerful or special. Just plain naive, thick headed and mediocre protagonist.