Nabu the Wise was a Lord of Order and cosmic being who arrived at Earth billion of years ago to take a proactive role in battling the Lords of Chaos. A cosmic being with near limitless magical power, his spirit resides in the Helmet of Fate and is the source of the great power hold by those whom bear the mantle, Doctor Fate.
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In the Young Justice cartoon, Nabu's spirit not only resides within the helmet, it also possesses whoever wears the helmet and uses his body because by himself he can't do anything. For some reason Kent Nelson seemed to have control over himself. But after he died, what happened is that Nabu possessed the next owner of the helmet and prevented him from taking the helmet off. In the episodes where Kid Flash and Aqualad had to resort to the helmet, Nabu only released them because the spirit of Kent Nelson apparently spoke to Nabu and convinced him to do so. When Zatanna Zatara had to wear the helmet to help her father Giovanni Zatarra. Giovanni told Nabu that releasing Zatanna would make him the new Doctor Fate, and that's what happened. In another episode, Nabu reveals that Giovanni didn't want him to do this just to free his daughter, but because now that the new Doctor Fate is a member of the Justice League, team members can keep an eye on him because Giovanni doesn't really trust in Nabu.