Before I Knew it, I Had Stolen the Dark Lord’s Heart

Seeing the relationship between Duke Willow, the Sword of the North, and her father who's his guardian knight, Celine has been looking forward to meeting her lord, Damian.
However, the Damian she encountered was a skinny, frail boy with a bright smile that belied his Northern Majesty.
To fulfil her duty as a guardian knight, she decided to take her lord's side, no matter what kind of person he was.
But there's something fishy about this frail boy…