If I use Challenges Only (after you opponent makes a control check that check gets -4) on a check that was less than 4 to start with does it reduce the check to 0 or into negative numbers? My opponents usually think that it should stop at 0 but I haven't found any rules that would imply that. This question also applies to ::: Akuma ::: as he has almost the same ability.

any number can be negative

mastermajora said:

Well, except for positive numbers. LOL.

What he meant was any statistic can have a negative value, be it control check, control value, difficulty, block difficulty, speed, damage, or even keyword abilities. About the only stats that can't currently go negative are Ht & Wt as there aren't currently any cards that change those (only reference them).