She adopted the hetaera life, and was the "companion" of Stilpo himself.
The Thais of that play is the most favourable delineation of the Athenian 'hetaera' in ancient literature.
The concubine has the status of a hetaera; she travels with the man, keeps his accounts, etc.
1820, "mistress," from Medieval Latin hetaera, from Greek hetaira "female companion," in Athens opposed to "lawful wife," and thus embracing everything from "concubine" to "courtesan;" fem. of hetairos "comrade, companion," from PIE *swet-aro-, suffixed form of root *s(w)e- (see idiom).