Styx (“Estigia” in Spanish) was a very important Oceanid (water nymph), who became the goddess/representation of the river Styx, the river of hatred of the Underworld. Her name means that: “hate”, duh
As a river, or lake, she is well-known as the way to enter the Underworld through Charon’s boat and for being the source of a man’s immortality (Achilles, for instance, who as a chubby baby went there to have a bath). As a goddess… well… She’s pretty cool. She sided with Zeus in the Titan War and, because of her powerful help and loyalty, he promised every oath will be sworn upon her. That’s the reason why, when a god swore something he/she should do it no matter what (Zeus’ promise to Semele is a tragic example, oh yes )
She was a daughter of Oceanus and Tethys, and the mother, with Pallas, of Kratos, Zelus, Bia… and little Nike!
This was requested by my sister sometime ago in tumblr, so there you go, sis (and everyone), hope you like it