Call me dumb but I didn't quite understand what leatherman was trying to say, or explain.
Is it that A-ko knew where and when to said "No" and doesn't let herself get carried by the flow of things, or that of other people?
I think he was saying that A-ko was a person who didn't let anyone know her true self. Like she knew when to cut people out/set a boundry when people got to close to protect herself. For example mc was friends with A-ko but the friendship seems to have been cut when she dropped out, when they meet again A-ko hangs out with them for a minute but ditches them soon after, and most importantly she showed the mc her sketchs but never her manga. Showing mc never fully knew A-ko because A-ko never really let her.
On the other hand the mc seems to wear her heart on her sleeve. She's sincere with things in her life. She's open with telling people she wants to be a manga artist, to being friends with A-ko, and she speaks plainly unlike A-ko.
The leatherman man was prob warning mc that people who wear their heart on their sleeve tend to be taken advantage of because they only see the good in people, don't read between the lines, and don't know when people are trying to hurt them. Also since they were in an underground event there were prob plenty of people there trying to take advantage of people.