My Sister Picked Up The Male Lead

I couldn't believe what happened before my eyes.
"Sister, what is 'this'?"
"It's your new friend, you said you needed a new friend no?"
One day my sister picked up a man.
It's astounding to just "pick up a man", but the more shocking fact is that this man...
In a palace battle, he lost his memory. By day he was a child but by night he was an adult man.
"I wish I was always in pain"
"Huh? Why?"
"Because Amy is always by my side"
Why is this guy dangerous?
In the morning, he's a cute and lovable puppy...