Hello,
A few questions about this combo...
1.) If the parasite cage is played as a "friend" to a Dark Player card, and his ability is used to take a friend from a Light player, can another copy of that friend be played by the Light character? I believe not, because one is still in play.
2.) Since parasite cage's ability will not return the friend taken by his ability unless parasite cage is defeated, if the Dark Player uses the Parasite Cage in a challenge, will the taken friend return? I would think not because the parasite cage would not be defeated, it would be the player that would be defeated.
3.) If the parasite cage is discarded by some other card ability, will the taken friend return to the player? I would also think not because again, he was not defeated.
Thanks again.