Playing co-op as Lucas and Reinhart, you defeated a boss.

This must be done on a major boss Rajani, Old Dapper Gent, Jeyne or the Reborn God. It takes an hour to rush to the point where you fight Rajani, then you can just use local or online multiplayer to have a second player play as the other male character you arent playing as (After 30 seconds of inactivity, a local partner will become AI and follow you just like before, but you will still be in co-op).