Because Gyun-Wool needed to practice kidnapping first. Plus spending more money for supplies and food is fun I guess ╮(￣▽￣)╭
Jk, but seriously, she really was nothing more than a plot device (if you can even call her that).
I'm actually lowkey disappointed that she was reduced to some random person. I hoped that maybe she was that dude who killed himself (imagine if he never died, but started to dress up as a girl instead). Especially with that name tag it all seemed to lead to that conclusion. Seriously, this could've been such a good plot twist but it just came to...nothing. Smsh.
So tell me, why was Hayan locked up with MC? Hayan doesn't seem to have any significant role for him to be locked up.