This was a fun one, was confused by the 'Keep' button as before I double checked the page and realized 'Space' was to hit the bars I just kept failing. This does seem like a really cool concept and I could see other coins like poison being added. Maybe for the bar you could add debuffs or other things that the enemy could cause to your bar.
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Thank you very much!
The "Keep" button is a "Skip" mechanic for the Ability Test part, you keep all your Offensive/Defensive tabs on the Ability bar for next turn (With your next turn adding more tabs with your new coins of course)(It's kinda like, "I want to keep them for now, to hit harder next turn").