User blog comment:B214/Question and Answers 4/@comment-24098614-20141206073044/@comment-73.178.133.185-20141207205507

Well... there are no official classifications for a six-winged Angel/Fallen Angel. We don't even know how the number of wings relates to ranking/classifications. What we do know is that Ultimate-class Fallen Angels such as Azazel and Kokabiel have 10-12 wings while Low-class have only 2.

Based on this, I would have to guess that the ranks/classes go something like this to make sense: Low (2-4 wings), Midde (4-6 wings), High (6-8 wings), Ultimate (8-10 wings), and Chief [Azazel / Michael] (12 wings). Since Akeno, like most of Rias' servants, should be stronger than their current Devil class (in her case Middle), then as Anon1 suggested it would make sense that she is equivalent to a High-class Angel.