Una is the only daughter of the King of Stormhold, and also the mother of Tristan Thorne. It is likely that Una is the firstborn female child of the King of Stormhold as her name translates to the feminine form of the number 'one' in Latin (and other such languages).
Una had a child called Tristan with Dunstan Thorne. She was a slave girl for witch Ditchwater Sal. The witch bound her with a chain on her caravan. Also she transformed her often in a bird with white and blue feathers. Her work for the witch was selling some products on the market in the capital of Stormhold. She could only be free, when Ditchwater Sal dies. At the end of the film Ditchwater Sal is killed by Lamia and Una is free, but lately after that captured by Lamia as a slave.
In the last scene of the film you see Una and Dunstan Thorne holding hands, so we see they are reunited.