The key

Latch key kids are kids who return from school to an empty home due to parents working a lot (since hirose is an only child he probably was alone a lot at home), hence they are given house keys to open the door for themselves and those keys are usually hung around their necks. I'm thinking it's a security thing for hirose or a cute reminder that his key now represents that he won't being returning to an empty home. :')

Ooh that means I was a latch key kid interesting