He knows you have "Crystal Heart" syndrome. Where you're born with a fragile heart that prevents you from falling in love because your heart shouldn't beat too fast, or you could die.

He makes sure you take your meds.

He's a tsundere.

He acts prickly but secretly cares about you.

He can't confess to you due to your condition, so he hides it.

He's rather stern and stubborn but secretly likes you.

He wants to keep you alive, safe, and happy before anything.

All guards have medical training.