Met through forced marriage

(Mostly through Tsukasa POV)
Two rich families - Tenmas and Kamishiros - have decided to set up their kids for marriage due to a good business opportunity, the kids being Saki, who's 16, and Rui, who's 17. However, as it turns out, none of the heirs were interested in this marriage, especially when they had different romantic interests. But none of the parents knew about it.

Tsukasa could've been sent off into his own father's business and get sent off to marriage instead of Saki, his younger sister, but was never interested in it since he wanted to become a musician. His parents never approved of him, and so didn't others. Well... except one person who was completely out of his engrossment who later, as it turns out, shares a lot of common things with the boy. Who could that be?

⚠️contains: underage smoking/drinking, suicide thoughts, self-harm, abuse, swearing, homophobia