The unusually named boss "Bizarro-Sephiroth" was a result of a mistranslation. The boss is actually known as "Rebirth-Sephiroth", but due to the letters "B" and "V" being interchangeable in Japanese and "th" often being translated as "S" (the same issue led to Aerith being known as Aeris), it ended up being translated as "Reverse-Sephiroth."

The translators then replaced the word "Reverse" with "Bizarro", which in the west can be used as a slang term to refer to something that is opposite or backwards. It is derived from the character Bizarro from the Superman comics, who was an imperfect clone of Superman who consciously did things the opposite way they were expected.
Contributed by EnoRed
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