Find the missing character?

The 4 diagrams as a whole contain 2 sequences of letters. The first starts at the top of the top left hand diamond, and goes vertically downwards, then up to the top of the top right hand diamond and down again. Letters in this sequence are written in alphabetical order, in steps of 4 letters at a time. The second sequence starts on the left hand segment of the upper left diamond and moves in horizontal lines from left to right, top row then bottom row, with the letters written in alphabetical order in steps of 5 letters at a time.