Find the missing character?

Starting with the upper left hand box of the left hand diagram, and moving to the right, letters follow the alphabetical order, 5 letters at a time. A second sequence starts from the middle of this row and goes down, with letters following the alphabetic order, 6 letters at a time. Repeat this pattern for the other diagrams, using steps of 6 and 7 letters, 7 and 8 letters, and 8 and 9 letters.