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Karellen, wishing to know the true nature of human kind (Love), puts Fei Fong Wong through one final test, this time against Myyah as the 'Urobolus', the final incarnation of her true self. In this form, she is half human and half serpent, with angelic wings. Both Fei and Myyah/Urobolus are naked in this fight, and Fei fights her in Xenogears. The music that plays is "One who Bares Fangs at God".
Once Urobolus is beaten, Karellen finally understands this meaning of Love between the two and humanity itself, something Sophia had tried to teach him long ago. Karellen then allows Fei and Elly return to their planet to live their lives with their friends. Although Karellen envies Fei and Elly, he continues his path into the Wave Existence to walk with God.
Urobolus is a misspelling of Ouroboros or Uroborus, a Greek word meaning "snake eating its own tail".
The symbolism intended by this is explained on page 171 in Perfect Works:
"The urobolus is a snake that eats its own tail. The self born from its own self, before psychological awareness is in a state of hibernation, a gathering of unconscious will. Again, it cannot make judgements, unable to tell the difference between self awareness and reality, thus keeping it in a cocoon. With the growth of self, it develops from non-consciousness to the need to escape the ring and live. Here, it creates a world of its own consciousness. When able to firmly judge reality, it starts to emerge from the cocoon as it surpasses it to enter reality. In terms of Myyah, this refers to the word expressing the idea of a hampering mother in the process of deriving the child from the ring (growth of self)."
Considering the Biblical overtones of Xenogears, and Myyah probably representing Eve, her serpent-like from may be a reference to the snake who offered Eve a fruit, and Myyah/Eve being tainted by the snake's influence as a result.
An interview revealed that both Deus and Urobolus are symbolic of the Demiurge, often depicted as a snake.