I think one of the reasons why Nam Do San can be a bit taciturn with low self-esteem — aside from being unable to reach his parents’ expectations — is due to what happened during his childhood. *spoiler warning* Nam Do San resorted to cheating, albeit unintentional, during the Mathematical Olympiad tests.
Nam Do San’s identity is closely linked to him being the youngest to win the Mathematical Olympiad for high school students, despite being only around 10 years old that time. We’ve seen this huge cardboard certificate being shown almost every time, and he was also introduced and got known for this title.
It’s possible that he finds it unable to keep his head high due to him thinking that he doesn’t deserve the title. Plus, not being able to open up about it to anyone for years took a big blow on him. In turn, he finds it unable to see his good qualities, and has often believed that he’s a “nobody.”
I can’t condone him for lying and cheating; what he did was wrong and he should take responsibility for his actions. But I’m glad that he finally got to tell the truth, and is seem to be finally starting to become more honest. Plus, the certificate getting trashed kind of signals a new beginning for Nam Do San. This time, he won’t be seen as the youngest Mathematical Olympiad, but for who he really is now.