100% Important
The percentage counter is the group of numbers to the right of the character's portrait in a match. It represents the damage dealt by opponents throughout the entire stock. Once you reach 90% or so, be careful, as you are now susceptible to be KO'd.
Percentage Decimals
It's worth noting that the game calculates damage in decimal, rounded off into an integer. So, say you have a fully staled projecile. At first, the projectile appears to be so weak it deals no damage, a few shots later the shot suddenly succeeds in dealing 1%.