Is there any character like that?

[Koen G]

I don't think so. The pairs or , ar , ol , al are all frequent, and it is also here that the continuum exists.

o is less frequent before i, but this occurs.
a is less frequent before k, t but this also occurs.

The contexts don't have to be exclusive. The frequency of aiin, ok, cho.. all point to a meaningful distinction between a and o. That's leaving qo out of the equation.

Maybe a and o are contextually "nudged" variants of the same?