Once upon a time, Jason Todd was a young, bright-eyed boy with a heart of gold and a happy-go-lucky attitude. Being adopted by the Duke was probably the most magical thing that could ever happen to a street rat like him. And for long enough all the boy wanted was to become a knight and do great things to protect his kingdom.
However, of course, that was once upon a time ago. After being caught and locked up in Arkham by the Joker, Jason was brutally tortured by that lunatic and his inmates. No words could possibly describe what he went through, the physical pain caused by various weapons and mental pain from cruel curses. By the time he finally broke free, that little naive squire had died, all that was left of him was a broken man with a twisted mind and a burning sense of revenge for the man who'd abandoned and replaced him.
Calling himself the Arkham Knight, Jason'd spent the next few years seeking vengeance, trying to kill Bruce and ruin everything that the Duke loved. However, he still lost, being bitterly defeated by his old mentor. And although Bruce'd tried to explain, to ask Jason to come home, the Arkham Knight refused, too blinded by anger and hurt to see through the veil of lies that the evil clown had injected into his brain. So he decided to run, run away from Bruce, from the guards, from Gotham, wanting to continue on his path of vengeance.
A few days later, as he was wandering about with a bleeding gash on his stomach, Jason stumbled onto you, a royal reading in the middle of a flower field. The scene would've felt like something straight out of a fairy tale, if his body hadn't gave out right after and he ended up collapsing right in front of your eyes.
You couldn't let him die, of course, so as your head flew into a panic, you hastily brought him back to your castle.
After waking up, all Jason knew was that his wounds were healed, he was in the neighboring kingdom, and you were his savior, even though you didn't know him at all. He didn't know how to feel about that, really.