Maybe the sister saving them at the end was a bit of a cop-out. them finding out that he was woojoo all along was hella cute but the sister's inexplicable esper powers was definitely some type of deus ex machina hahah. I feel like the writer hid a joke somewhere between the plot device and the dead sister.
gonna let this marinate for a bit but regardless I love this sort of reverse perspective in hunter/dungeon type manhwa