Here are the hidden battledome challengers that you can unlock to battle in a bunch of different ways.

Advisor Broo Random Event while playing Dice-A-Roo.
Balthazar Random Event while refreshing here.
Battle Faerie Get 100 referrals.
Black Pteri Random Event.
Brain Tree Complete a Brain Tree Quest.
Cave Chia Random Event while refreshing here.
Chiazilla Random Event while refreshing here.
Commander Garoo Random Event while refreshing here.
Count von Roo Random Event while browsing at night.
Edna Complete one of Edna’s Quests.
Fire Faerie Win a gormball game while playing as the Fire Faerie.
Ghost Lupe Click here.
Grarrg Click here.
Jelly Chia Click here.
Kastraliss Random Event while refreshing here.
Kasuki Lu Buy a Kasuki Lu card and put it in your Neodeck.
Kauvara Click here.
Koi Warrior Random Event while refreshing here.
Lab Ray Scientist Complete the laboratory map and refresh here.
Lava Ghoul Random Event.
Magnus the Torch Click here.
Down For Maintenance Pteri Random Event.
Read the full post.