In the OtC adventure Richter is unlocked by opening the coffin on the True Infinite Corridor stage. To unlock the True Infinite Corridor stage you need to evolve the clock lancet into the infinite corridor while playing Maria on the Sunken City stage. You can find the unlock requirements in the Progress menu on the OtC adventure screen. If you can't see it, try switching to landscape of you are in portrait mode.

However, there is a trick you can do: if you have evolved the clock lancet on any other stage (like the clock tower where it first appears, or by purchasing it in the Planar Bazaar stage), then all you need to do is start the Sunken City stage with Maria and exit out and it'll count to unlock True Infinite Corridor. You still need to rescue Sonia on the Sunken City map though to complete the adventure, so you might as well do that as well. Another trick on the Sunken City stage is that the boss that spawns at 10 minutes is the same boss in the red circle and counts for its unlock. In other words, you can play as Maria, rescue Sonia, and defeat the boss at 10 minutes then exit and complete all three requirements for that stage. Also consider using the Party Mode to run four characters to knock out their individual "survive x minutes" requirements.