@Soriku
Since you've finished Yukiko's Castle, you can now access the Bonus Boss the next time you go to the world inside the TV. He's 10x tougher than Shadow Yukiko, and you need to be at level 21 at least to have a chance against him, but it's worth it since each bonus boss gives you a secret weapon for one of your characters. This one gives you a Fan for Yukiko.
You may skip the Bonus Boss battles since they are only optional, but if you're a completionist, you may want to take the challenge ;)
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
