What should be the next letter in the following series?
a z e b i y o _

The letter C, because the pattern is this (I think):
Every two letters it states the vowels (a,e,i,o,u)
Then in between every two letters there starts a pattern of, saying the alphabet in order and saying the alphabet backwards.

In other words they merged these two lines:
A, B, C, D, E...
+
Z, Y, X, W, V...
=
A, Z, B, Y, C, X...etc.
then by adding this pattern of vowels you get:
A, Z, E, B, I, Y, O, (C), (U), (X)...