JenovaJenova is a world-devouring alien, which descended on the Planet many eons ago, possibly around the time when the Cetra began disappearing. He/she ended up trapped in the frozen Northern Crater, until some Shinra scientists discovered her and thawed her out. At that time, Shin-Ra mistakenly thought Jenova to be one of the last remaining Cetra.
Sephiroth was a product of Jenova's being infused into a young fetus. Fortunately it seems the child's strong genes overcame the Jenova cells, and he was able to control all of them. Sephiroth later learns of his relationship to Jenova, and incorrectly thinks she is both a deity and his mother.
Final Fantasy VII
Final Fantasy University
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