Stats are very important in the Chao Stadium. There are six grades: E, D, C, B, A, and the best, S. You can check your Chao's stats by bringing it to the Chao Doctor, but you don't have to! You can tell your Chao's stats by giving it some animals or Chaos Drives and seeing how much the stat raises. Some say you can't tell, but through testing on my own game, I can tell you that it is true. So, go gather up a few Chaos Drives of the same color (they are the easiest) and hand them to your Chao, when it gains a level, look for the number that appears on screen and compare it to this chart:

Most Likely is:
+26, +27, +28, +29, +30 = S
+23, +24, +25, +26, +27 = A
+20, +21, +22, +23, +24 = B
+17, +18, +19, +20, +21 = C
+14, +15, +16, +17, +18 = D
+11, +12, +13, +14, +15 = E

Definitely is:
+28, +29, +30 = S
+25 = A
+22 = B
+19 = C
+16 = D
+11, +12, +13 = E

To raise your letter grade all you need to do is evolve your Chao in the right alignment. This means, if you want to raise your run grade (for example), evolve your Chao into a run type Chao. So getting an all S Chao will take a lot of evolving and reincarnating!