I'm really confused about the Hayabusa Spirit because from the
Ninja Gaiden Wiki this is what it says:
Hayabusa Spirit: This is the guardian animal spirit that helps resurrect Ryu after he dies from being sliced in half by the Dark Dragon Blade. Its ethereal spirit form is seen merged within Ryu, its corporeal physical form can be seen after Ryu disappears into the night. Ryu sometimes drops a few peregrine falcon feathers in his wake. It serves as the force of Ryu's Indomitable Will, as he is able to overcome any challenge presented.
This is from our Wiki "Using this, Ryu can tap into and unlock the limitless potential to do nearly anything he sets his mind to. The Hayabusa Spirit grants Ryu the power to do feats such as Stopping the flow of time, Creating Black Holes from nothing, and even jumping thousands of feet in the air, and much more"
And from the
Ninja Gaiden Sigma Plot:
Ryu attempts to save his best friend Kureha, but is unable to reach her in time and was forced to watch as she is killed right in front of his eyes by the Greater Fiend Doku, who then proceeds to behead senior Hayabusa ninja armed with the Dark Dragon Blade, a relic his clan is tasked to protect. The two square off but Ryu is quickly overcome by the Greater Fiend and the Dark Dragon Blade. Ryu collapses and seemingly dies. Hours later, the Hayabusa animal spirit appears, the peregrine falcon landing on a tree above Ryu's body. What transpires at this moment is unknown.
Three weeks later, having fully healed from his wounds and informed by Murai of the mastermind behind the attack on his village, Ryu Hayabusa, donning the Black Falcon attire, makes his way towards the Vigoor Empire, an isolated landlocked country with few ways of getting in.
Ninja Gaiden Sigma: Story Mode Playthrough Part 2 (Go to 17:00) The description also has a summary of the story