itch.io

Will there be a Linux build or will it be Windows and Mac only?

It does have an effect and you should be able to see it on the side, with the other effects applied to you (can't remember how it's called).

It's however unclear how much is needed to progress to the next step. I don't know if the special skill is necessary or if any mean to make him grow works too.

When you are in the city/camp, you have a menu with a sword icon. Select it and you'll then have to pick 6 moves. Once you have, select the Apply button at the bottom.