It's in REVERSE creation order of the accounts owning the characters, at least for me. No way to change it as far as I know (except for swapping the names of characters for $$$). Here's the order I created my accounts/toons in:

Shalph
Sheta
Shamma
Shelta
Shepsilon

(for the first 5 letters in the greek alphabet).

Here's how they show up in my main toon (Shalph) party:

Shalph
Shepsilon
Shelta
Shamma
Sheta

which is reverse chronologic order (and not really the order I wanted!). There's some info on this in the Wiki, I believe.