A Cook, A Doctor, A Ghost and A Ronin.

Explanation

"Haki? Was that the thing you used to break the boulder?" Inosuke tilted his head. So that's how he was able to break that rock! Can I do it if I mastered it too?! He is excited to learn Haki.

"Pretty much yes. And I am going to teach you how to wield it." Zenitsu lets out a sigh of relief. This time, they didn't have to go through such heavy training session. All of them are starting to get curious about the existence of Haki.

"What are we waiting for? Hurry! Teach us how to use it!!" Inosuke persuaded and hurry Law to teach Haki to him. He can't wait to use it. He wanted to become stronger than ever before. Law shakes his head in return.

"Not yet. There are some explanations that I need to tell you guys first." Inosuke looks displeased, but he gets along with it. 

"Before anything, do you guys have any questions?" Law asked all three of them if they have any questions regarding Haki. Tanjiro raised his hand.

"What is it Kamado-ya?" Law asked Tanjiro. The three of them are sitting on the ground while Law is sitting on top of the cracked boulder that de destroyed before.

"What is actually Haki?" Tanjiro would like to know in detail about Haki.

"Good question. That's exactly what we are learning today." Law get up from sitting on the boulder. 

"Haki is a mysterious power that allows the user to utilize their own willpower for various purposes." All three of them focused on Law giving explanation.

"All living beings in the world are capable of learning Haki. However, most of people do not notice it or fail to awaken it." Law continued. 

"Haki is a power that lies dormant in all the world's creatures... "Presence", "fighting spirit" and "intimidation"... It is not different from the things that humans can naturally sense such as these..." Law explains that every single creature that exists in this world had possibility of learning Haki.

"Do you understand?" He asked them if they actually understand his rambling. Zenitsu raised his eyebrows with a confused expression. Tanjiro gave Law a thumbs up. Steam could be seen coming from Inosuke's boar mask. Clearly that the information is too much for someone of his caliber.

"Basically, it is manifestation of willpower?" Tanjiro sums it up. Law nodded in agreement.

"How is that even possible? Using willpower and manifested it into reality?" Zenitsu find it impossible to do such thing. How can your willpower assist you on breaking a huge boulder with one punch?

"I don't know, I just learn it." There's no reason to learn behind the mystery of Haki. It's just proven to be a great waste of time.

"Where does it originate from?" Tanjiro asked Law another question. 

"Haki originates from an individual's spirit. Like I said before, it's the manifestation of your willpower and determination" Law answered Tanjiro's question.

"There are three types of Haki. But I'll teach you guys two of it." Law planned to teach them on how to use Armament and Observation Haki. Unless they have the potential of using Conqueror's Haki, he will only teach two of them.

"There's Armament Haki and Observation Haki." Law pointed out that there's two types of Haki that they will be learning.

"What's the difference?" Zenitsu asked for Law to explain it further in detail.

"Armament Haki allows the user to use their own willpower to create an invisible armor around themself, providing incredible offensive and defensive capabilities." More smoke came out from Inosuke's mask.

"In other words, ability to harden any body parts as we desire" Tanjiro simplifies it to help Inosuke understands it.

"It is as I just showed it to you earlier." Law pointed at how he destroyed the boulder with the help of Armament Haki.

"Then there's Observation Haki. You know what? This one is easier if I show it to you guys rather than explaining it." Law pulls out a blindfold from his pocket. He tied it around his eyes, blocking his vision.

"What are you doing, Law-san?" Tanjiro wanted to know as Law walked towards their usual training ground. He opened his arms wide.

"Throw anything towards me." All of them are confused.

"Are you sure?" Zenitsu didn't like the idea of doing that. It's not like he really wanted to do anything anyways. This guy got a loose screw on his head!

"Gyahahaha! As you wish!" Inosuke stood up and laughed. He took on the challenge and throw his swords towards Law in incredible speed.

"Wait a minute! Think about it you buffoon!" Zenitsu shouted at Inosuke to try and stop what is he doing. But he was too late.

*Fwoosh!*

Law stood still, before he simply just recalibrate his position in a few seconds while blindfolded with Inosuke's blades passed over him. He didn't show any fear at all. To the point he managed to even grab Inosuke's blade by the tsuka. Both of them.

All three of them are impressed by Law's bold action. Law had no reaction to that at all, other than putting the blades away. He brushed off his hands before explaining them what had happened.

"What did you do?" Inosuke asked how did he able to catch those swords, while blindfolded.

"That...is Observation Haki." Law took down his Observation Haki before tossing it over to Tanjiro.

"It allows the user the ability to take notice on other's presence, strength, and emotions. You might as well call it as sixth sense seeing from how it actually works." Law explained.

"Is that even possible? To sense presence of the others?" Law nodded to the question.

"Most importantly, which one would we learn?" Inosuke is excited to learn Haki. He wants to learn more as he started to get very interested in Haki.

"Here" Law takes out two more blindfolds and toss it towards Zenitsu and Inosuke, which they catch.

"Put it on. From now, we will train on how to awaken your Observation Haki." Law cracked his knuckles, telling them to wear the blindfold.

"I did promise that I won't hold back, did I?" Law lets out a devilish grin at the end. Tanjiro gulped his saliva. He could smell a scent from Law, a scent that is full of malice.