Oikawa only knows y/n aka Mikuni as his sister but didn't know that he has another sister (Tomoe) that's a secret who's is living her own life and dating his best friend (Iwaizumi) behind his back. And he doesn't know that mattsun and Makki are together either about kindaichi and Kunimi being together, kyotani and Yahaba being together cause they are doing everything behind his back.