One main sad moment was when Ash was separated from Pikachu. They are such a close pair, and being apart was really tough on them. It made Pikachu feel lonely and Ash felt like he was missing a part of himself.
Time is something that catches people off guard. The past has passed, the future has come, and only the present remains.
Those people who I thought would be together forever gradually disappeared from each other's lives. No farewell, no goodbye, only wishing you happiness.
When a special person appears in your life, you will, you will, you will start to live a different life. This is the meaning of love.
Leaving doesn't mean it's over, nor does it mean you can't start over.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
When Marinette has to constantly keep her identity as Ladybug a secret from her friends and family. It can be really sad as she has to lie and hide things from the people she loves, like when she misses out on normal teenage activities because she has to go save Paris. She feels isolated in that aspect.