This is a brush i use to create border in all my comics! I also sometimes use it for text bubbles.
It creates a slightly wavy line that look like it was drawn manually :D
I always use A4 page format, and the size of my brush is always 40!
It couples well with a lineart drawn in THIS BRUSH (https://assets.clip-studio.com/en-us/detail?id=1990141) size 15