Essi Daven is a talented bard (also known as a "trobairitz") and also a friend and professional rival of Dandelion. The latter has always treated her like a younger sister. She is quite possibly the only woman Dandelion had not actively tried to bed. She appears in the short story Trochę poświęcenia (A Little Sacrifice) in Miecz przeznaczenia (The Sword of Destiny) collection by Andrzej Sapkowski.
Her nickname in Polish (Oczko) translates roughly to "Little Eye" or "Eyelet", but this is likely used jokingly as her eyes are described as giving meaning to the phrase "eyes like stars in the sky" and of a magnificent, riveting, dark blue. One eye is constantly hidden by unruly tresses. She wore a silver flower-shaped pendant with a blue pearl.
After a brief romance with Geralt, she parted ways with her friends, Geralt and Dandelion.
That was the last time she and Geralt were together. Four years later she met her death in Vizima due to smallpox. Dandelion himself took her body through piles of burned corpses and gave her a proper burial in a peaceful place, while also burying her with both her lute and a trinket that Geralt had given her 4 years prior which she hadn't take off since. This is related at the end of "A Little Sacrifice", when the narrator tells us the true history behind the Dandelion's new ballad. Which was never performed.
The journal entry for Hounds of the Wild Hunt contains a quote from Essi Daven, but her nickname is listed as "Blackjack".