The Ninja Slayer Extras: Headcannons and Drabbles

Ninja Headcannons!

The reason First Ninja smoke bombs around so much instead of walking is just because he doesn't have to replace his smoke bombs anymore. He's basically a ghost living in a book with free access to unlimited Ninja weapons and an eternity of freedom. He doesn't need to replace any of his gadgets. He can use any Ninja tool whenever he wants so long as he doesn't start plotting evil schemes or something. 

Other spirits get to go to the afterlife.

First Ninja decided to be the mentor of the Ninja.

The least the Nomicon could do was let him be mysterious and make it so he doesn't have to walk. He's dead! He can have a little fun! He doesn't even have to walk anymore! It's so much cooler this way! (He wants to impress future Ninjas (Mostly his son aka Randy) and this is the only way he can think of doing it besides leaving the Nomicon.) First Ninja deserves unlimited smoke bombs. As a treat. :)

Can First Ninja even leave the Nomicon? 

I imagine it's sort of like how text and graffiti from the Nomicon can appear in the Ninja's vision while they are out of the book. He would appear mid-fight to give advice or tell them that the Nomicon really needs to see them. He wouldn't show up as often as the Nomicon but that's just because he holds more urgent information. Both types of information but First Ninja's information is always on a time limit.

What's the time limit? Who knows! But you better figure it out fast.

Also why did First Ninja take so long to appear to Giyuu? Giyuu was the Ninja for 7 years before Randy showed up. But First Ninja didn't appear to Giyuu until then. Why?

Because the First Ninja in Giyuu's version of the Nomicon (from the past) didn't believe that they had to. The thing that Giyuu is best at is following orders (should they not break his moral code, which the Nomicon's rules didn't do) and he's never posed any issue for the Nomicon. So the Nomicon saw no point in sending First Ninja.

Giyuu was very good at hiding the Ninja. Nobody who saw Giyuu often ever saw the Ninja before. Lower ranked slayers did sometimes and the Ninja was regarded as a cryptid or spirit protector for the corps. (Murata has seen the Ninja before and briefly suspected it was Giyuu but never mentioned it to anyone)

Now the last question of the chapter!

How did Zenitsu manage to sneak up close enough to Giyuu that he figured out there identities?

Ninja senses can extend for miles! How is it possible that Agatsuma Zenitsu was able to hide his presence from Giyuu?

Randy obviously got a pass since he was asleep.

Short answer: Giyuu was distracted while carrying Randy back to the Water Estate.

Long answer: Giyuu was in his own thoughts as he was carrying the sleeping Randy back. And then Shinobu came up to him right after he shut the door and he was tired from the previous battle. And 'Oh God I think I've just emotionally adopted a child.' And... he just forgot to enhance his senses. Forgot that someone could have followed him. And truthfully, he had only met Zenitsu once before. He didn't even know the kid had advanced hearing. 

Zenitsu will brag about that until the end of time. It will even be in his autobiography!

(In case you didn't know, after the final battle and Muzan's defeat Zenitsu becomes an author and writes about his adventures as a demon slayer.)