Character Classes

[MarcoP]

It would seem that a and y are most similar because they both appear often after d. I find that outcome acceptable as I consider them dependent on the following characters.

Yes, that's basically the only thing they have in common, but it's a quite relevant feature.
a / a and y / y are the only two characters that often follow d / d, and they follow d with similar frequencies (da occurs in 30.2% if a-words, dy in 31,5% y-words).