The next letter is: S
Explanation:
There are two series in this sequence (YBWDUF), which are as follows:
1. The first series is YWUS... in background direction by skipping one letter. Like Y(skipping X)W(skipping V)U(skipping T)S. (Hence S is the next letter)
2. The second series is BDF... in forward direction by skipping one letter. Like B(skipping C)D(skipping E)F and so on.
Both of series are in alternate positions as follows: YBWDUFS.
