Improves reflexes (I think)
It doesn't improve math skills at all (unless the player's in kindergarten or something); rather, it strengthens the reflexes to calculate single digits (and in totals not less than 1 and exceeding 9). This game's actually quite useful to build up hand-eye coordination and dexterity with the Number Pad (the latter is good if you're entering data into spreadsheets rapidly or something like that.)
In fact, it's a good thing you implemented the number pad controls, as I felt that mouse control to click on number buttons is way too slow.
I felt that the stats system is there to influence how fast each match lasts. By emphasizing attack, matches are very quick-you deal so much damage, but so does the opponent. By emphasizing defense, matches are extremely long, more like an endurance round. Luck and the random critical hits to go with it probably don't have a place in this game, but that's just me.
As for the art, I didn't really care, and neither should anyone, as the player focus should be solely on the computations. (Look somewhere else for pretty graphics-you won't find them here). An option to change the background color and the line colors could be useful, as staring into a mostly black screen can be depressing.
The sound effects are only needed to confirm hits-so ideally they should be different when the opponent hits or when the player hits.
My main gripe is that the minimum damage inflicted on either the enemy or player should be set at 1, not 0.
Also, what's the point in locking the attributes in after each match. Wouldn't It be better if the player gets to redistribute all of the total earned points as one sees fit before each match.