お兄さんは心配性 (Onii-san wa Shinpaishou)

Background
"Onii-san wa Shinpaisei" is an original KAITO song. In it, he laments his fears of his sister being taken away by another boy, when he always "loved her from the start". Constantly doting on her, spying on her and doing other questionable actions to keep tabs on his "dear little sister", she thankfully doesn't put up with it very often and puts him in his place. Though it doesn't deter him for very long, opting to risk his wellbeing to ensure nothing happens to his sister.