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Blue by Billie Eilish is Touya Todoroki's song.
"Colder than your father's eyes, he never learned to sympathize, with anyone."
This line could honestly go for Touya or Enji in the aspect of never sympathizing with anyone.
"You were born reaching for your mother's hands, victim of your father's plans, to rule the world."
"Too afraid to step out side, paranoid and petrified, of what you've heard."
It explains him so well. And these lines:
"I don't blame you, but I can't change you."
This one reflects the understanding that the rest of the family may have towards Touya's actions that were based on his father, such as getting away from the house in general.
"I don't hate you, but we can't save you."
Could symbolize the relationship between him and his siblings, or even Rei. The song itself is very well written, and the melody and pitiful tone in which the lines are sung at would push even further into the idea that this song explains him well.