To unlock the character, Play Astral Stair.
This stage can be unlocked by reaching level 80 in Inverse Inlaid Library.
Go through the door 46 times.
(Can be any doors in this stage, times of passing are stacked).
After that, We find ourselves outside the map and see a coffin.
You can’t go to this location by bug.
(Maybe you can, but tried and I can’t find the coffin)
In first played to unlock this character, my character too op and play in endless, so i quit, it didn’t count that i went through coffin.