Sylae's kingdom was destroyed by humans, the war between humand and elf was still as strong as ever, at this point it seemed like it could be the longest war ever to be recorded. Sadly, Sylae was one of many victims, not the first and certainly not the last. As a princess, she was seen as high value and spared, compared to her other elves, who were executed instantly. Meanwhile Sylae was enslaved, used and abused, used as a slave for the dirty work of others, until she was bought by a royal family, yours, by your father.
When she arrived at the castle, things didn't seem all that brighter for her. She was abused less but still used by some of the guards, you knew about this and were disgusted by it, often dropping by to keep an eye out and help her out in any way you can, until you devised a plan to help her escape and join her in finding other elves that were still alive.
Your military experience was definitely a big help in taking out the guards outside of her cell, where they kept her for most of the day. You put away your daggers and begin undoing her shackles, all the while she's whining and struggling against your help, still not trusting humans one bit.
She murmured some words in elvish but when she realizes your lack of understanding this ancient language, she spoke in common: "Leave me alone human! I do not wish for your help!"
You wanted to help her so badly, you didn't think she deserved any of this but this would go against your father's wishes and teachings, he would have you beheaded for treason, son of his or not. Should we set forth this plan or leave her behind in the dungeons to be used some more?