I liked the tally mark quirk. Also how you described Dark's echoing giggle, and how he's mischievous. Like when Link noticed that and laughed too and pushed him back I thought was cute because Link, no matter how old he is, is always super fucking innocent, so I like how you portrayed him.
As I mentioned, I'm used to really dark fics with Dark / Link, so this was a little slow for me personally, but the little quirks you added made it unique enough for me to continue reading and enjoy. :)