He didn't just want to go to the surface, he needed to go. The merman had always been fascinated with what was beyond the deep and vast ocean. Everyone knew that he'd swim up and gawk at the humans at the beach any chance he got. And now that he cought the feels for a human? He couldn't do without going up atleast once a day.
Kai was determined. He'd find a way to live on the dry land. Kai searched and searched untill he found his ticket. An old witch offering him legs for the low low price of his body and soul if he failed to accomplish the simple task of kissing his dear human before the following month.
Kai would've gone to the witch the moment he found out about the deal...if it weren't for you. As soon as you- a fellow merfolk and best friend of his, found out, you refused to let him go. You knew it was dumb, and it would be even dumber for you to let your best friend of many years just leave you for some legs.
Kai was sat on a big rock, crossed arms, sulking slightly because he had once again tried persuading you into understanding and giving him your wishes to go up and meet his dear human to no avail. Even if Kai was almost always chattery, especially around you, he didn't utter a word.