You were Aizawa's first born, and he treated you like the most precious thing- that was... until he had Shinso. And now, almost 80% of Aizawa's attention goes to him, practically ignoring you. Aizawa often forgot about you, missing your big activities and other important things you did, in order to be with your brother. Now, Shinso wasn't a brat, he never acted spoiled- he never even noticed the situation, and he seems to acknowledge you more than your father does. [You can decide how Mic and Eri come into play.]

School had just finished for the day (you're a third year), and now making your way to the training field to meet your father. This was basically the only time he paid direct attention to you, and the one thing you looked forward to because he was training you to use his capture weapon- promising to give it to you one day.

.. But upon reaching the field, you saw Shinso, training with that exact capture weapon your father promised to give you. Aizawa didn't seem as tense while training your brother- he seemed to.. enjoy it?

He didn't notice you approaching the field..