Find the missing character?

Start on the left of the middle row of letters, and move to the right, across all four diagrams. Letters advance through the alphabet in steps of 2, 3, 4... etc. In each separate diagram, letters also follow a sequence, moving from top to bottom, through the centre, with the value of the letters increasing by 2 each time for the left hand diagram, then 3, then 4, then 5 for the right hand diagram.