Getsu Fuma Den is easily the most obscure third party series in Smash. The only ones that even come close to being as obscure as it are some of the SNK series like Athena (although that’s not as obscure given its connection to Psycho Soldier which is part of King of Fighters.
It’s also really obscure when it comes to Smash itself, since the series was just randomly added to the Dracula’s Castle stage despite not being part of the same series as Castlevania, and wasn’t even mentioned in Simon and Richter’s reveal (unless I just forgot about it). Many fans, including myself, didn’t even know it was in the game when Ultimate came out.
It’s also really obscure when it comes to Smash itself, since the series was just randomly added to the Dracula’s Castle stage despite not being part of the same series as Castlevania, and wasn’t even mentioned in Simon and Richter’s reveal (unless I just forgot about it). Many fans, including myself, didn’t even know it was in the game when Ultimate came out.