So for those of you who haven't noticed, skills have levels. Skill leveling increases your Ögis damage/healing effect
For upgrading skills you just need to upgrade the character with cards of the same type, for example, to upgrade Law's skill, you should feed him with other sawbones units. You can choose to either level him up with units of the same type, or choose to wait for the max. level, because if you then click on "upgrade", it'll automaticly add all units of that type that are lower then blue. At the left bottom corner, you can see the chances of upgrading the skills.The more cards you use the higher the chances for it to be succesful. But at a high skill level, it doesen't really matter. If you reached that, use always only one Unit, because the chances will always be the same no matter how many units you use.
Note: It is unclear if "Low" means the chance is fixed at 1% or if it means that the chance is somewhere below 1% but increases as you put more cards in.
Note: skill levels do not stack or upgrade multiple times if you use high tier cards or low tier cards.
The chances for upgrading the skill go in this order:
Low -> Very Low -> Guarantee -> High -> Very High