Who is Diomache from Gates of Fire and what is their importance?

Diomache is Xeones' cousin. When Xeones goes to Sparta, Diomache chooses to go to Athens instead, where she hopes to find a good man and get married. This does not, however, work out, and she becomes a priestess in a temple. Diomache delivers Xeones' ashes to Sparta and visits Thermopylae after the Greeks finally defeat the Persians.