http://www.mangago.zone/read-manga/horimiya/
The boy has piercings, not tattoos, but other than that it sounds similar to this one.
hiromiya?
Horimiya?
wait what? :0
YES THANK YOU
THANK YOU EVERYONE
actually he has tattoos and ppeircings
okay so theres this manga ive been looking for it forever its about a girl whos all dressed up nice at school and theres also a "nerd" at school but one day her little brother is protected by the same "nerd" who turns out not to be a nerd (also has tattoos) then he sees her like how she would look like without being all glammed up and they keep those a secret.