The series B, E, D, G, F, I, H moves forward and backward by turns along the alphabet. What is its next letter?

The steps alternate: three places forward, then one place back. B to E is forward three, E to D back one, D to G forward three, G to F back one, F to I forward three, I to H back one. The next step is forward three, and H stands at place 8, so 8 + 3 = 11, which is K. Moving forward two gives J and forward four gives L; going back one again, as though the pattern did not alternate, gives G.