She was the daughter of Io and Zeus; elder sister of Epaphus; and mother of Byzas, founder of Byzantium, with her uncle, Poseidon.
[1][2] According to the historian Hesychius of Miletus,[3] as Io, changed into a heifer and being chased by a gadfly on behalf of the jealous Hera, was passing through Thrace, she gave birth to a girl, Keroessa, on the banks of the Golden Horn, by the altar of the nymph Semystra.
[4][5] Keroessa was reared by Semystra and grew up surpassing other local maidens in beauty.
She had intercourse with Poseidon and in due course gave birth to a son, whom she named Byzas.
He became the founder of Byzantium (today Sarayburnu, where Topkapı Palace was built) and named the Golden Horn (Greek Χρυσόκερας) after his mother.