Why is Ken so cheap?

Compare Ken's pattern in column 4 vs other characters. Column 4 must be cleared to allow more blocks to be played. Consider full pattern and half pattern x2 attacks.

If Ken drops his full pattern, you require 4 different color breaks. If Ken drops half pattern x2, you need 2x2 (r, g, r, g) color breaks.

Full    Halfx2
4       2x2     Ken
4       2x2     Devilot (70% damage)
4       2x2     Akuma (70% damage)
2       1       Donovan
2x2     2x2     Sakura
2       1       ChunLi
2       1       Morrigan
2       1       Hsien-Ko
2       1       Felicia
1       1       Ryu
1       1       Dan

There you have it really, Ken's pattern attacks column 4 as hard as Devilot and Akuma, but without the damage penalty.

Ken's Pattern:

Ryu's Pattern (note the easy to attack column 4):

You might be playing puzzle fighter hd, which seems to have had balancing changes according to this article. If the article is correct, much of this column 4 analysis still holds:

1. Ken still better than Akuma and Devilot (just a little less so).
2. Ryu brought up into the rest of the pack, now is better than Dan.
3. Hsien-Ko now as good as Ken (aka Overpowered!)