notice Werewolf and Minotaur couldn't be fought by Richter either.
it's because boss battles liek Death are spawned/triggered by a story cutscene first. for consistency purposes they decided to cut out the story cutscenes for these extra characters and therefore, there is no trigger to start up those boss fights in those rooms.
and without the trigger, those bosses can't appear.
yeah from the looks of it, technically, the extra characters in SOTN are more of a hack instead of a completely redone mode by itself (notice besides controlling a different character and status gauge, everything else is exactly the same, minus the cutscenes are bosses linked to those cutscenes) , they simply trimmed out the cutscenes and didn't bother to reprogram the boss fights that were only triggered after a cutscene.
so this is why Richter and Maria can't fight bosses like Werewolf+Minotaur, Succubus, Richter, and Dracula.
the only exceptiuon is Shaft but I guess in the original PSX, they decided to at least reprogram that boss so you fight him without needing a cutscene to tie in his entrance, otherwise, there would be no last boss to even lead to an ending and credits.
so that proves that the other bosses that can be fought COULD have been programmed to be fightable without needing the cutscene to trigger them, I guess it was just due to time constraints back then (cause that was teh very reason why Maria wasn't playable in the original), and even now, I guess they didn't wanna bother making them fightable in the PSP version either.
but yeah, the Creature is still fightable, but he's a total joke really, I don't blame the video person for not recording that fight. that fight was almost as pathetic as the mummy fight.