This chapter is very similar to what happens in the Naruto series. But I thought it was vitally important to highlight the tree-climbing training. It will be a boost for Bakugo so he doesn't tire out too quickly.
All characters and story belong to Kohei Horikoshi and Masashi Kishimoto
"Can someone tell me what the hell just happened?" Bakugo asked impatiently.
—Sasuke rescued you when Zabuza's clone was about to stab you with his sword, after you lost consciousness. —Sakura explained. To which Bakugo, incredulous, looked at Sasuke, and the dark-haired man looked away, somewhat embarrassed. —Also, your attack made Zabuza drop Kakashi-sensei. Afterwards, Kakashi-sensei copied one of his techniques and defeated Zabuza.—
"And that bastard is still alive?" Bakugo asked.
—Well... —said Sakura doubtfully
—That's exactly what we were talking about just now, Bakugo. A hunter-type ANBU from the Hidden Mist Village came to finish him off, but I think...—Kakashi tried to explain but was interrupted by Bakugo.
"What the hell is a hunter ANBU?" Bakugo asked.
"They're elite ninjas, also known as corpse-killers. They destroy any trace of the corpses of deserting ninjas. A ninja's body contains secrets about chakra and their jutsus, which is secret information from their village. To prevent ninjas from other villages from learning these secrets, hunter-ninjas hunt down shinobi who have betrayed their village, then eliminate them and erase any trace of their existence," Kakashi explained.
"So he's dead, right?" Bakugo said.
"Most likely. But my instinct tells me no. I think the hunter-nin and Zabuza were working together. If he really was a hunter-nin targeting Zabuza, he would have disposed of the body immediately. However, he took the body elsewhere, jeopardizing the mission. Furthermore, he used the Thousand Rods to kill Zabuza. It's a weapon that can simulate death if applied to certain points, like the neck," Kakashi finished explaining. "So we must prepare for the worst. We must assume that Zabuza is still alive."
"It can't be!" Tazuna exclaimed worriedly.
"But Kakashi-sensei, you still have a week left before you can fight again. How will we face that monster?" Sakura asked worriedly.
—Heh. Next time I'll finish him off. —Bakugo said with a cocky smile.
"That's if you don't faint first," Sasuke said ironically.
"You bastard..." Bakugo said, clenching his fist and forgetting that any movement of his arms caused a sharp pain. "Fuck, that hurts!"
"Do you know why you fainted, Bakugo?" Kakashi asked.
—Because I used too much chakra. —Bakugo replied.
—Not exactly. It was because you wasted too much chakra. All three of you need to control your chakra. Especially you, Bakugo. I'll teach you this week—said Kakashi
—But a few days of mini-lesson won't make us defeat Zabuza. You were barely able to defeat him with your sharingan!—Sakura exclaimed.
—Exactly, it was because of teamwork that I was able to defeat him. You and Sasuke protecting Tazuna allowed Bakugo to go all out to free me. Together we defeated him once, and together we'll defeat him again. —said Kakashi.
"Stop the chatter and let's go train. I want to get stronger," Bakugo said.
—Mmm, we better wait until tomorrow for your arms to heal. —said Kakashi
"Heh. Tomorrow's going to be a fun day," Bakugo said, looking forward to the workout.
"Nothing's going to be fun," replied a boy in a hat.
—Huh? Who the hell are you, brat?—asked Bakugo.
"Inari! Where have you been?" Tazuna asked, hugging his grandson.
"Hello, Grandpa," Inari replied.
"Inari, say hello to the guests," Inari's mother suggested.
"Mom, don't you see these people are going to die?" Inari said, pointing at the four ninjas.
"The only one who's going to die here is you if you keep talking nonsense!" Bakugo replied, a vein popping out in his forehead.
"You don't stand a chance against Gatoh and his men," Inari said grimly.
—Just so you're clear, brat, that Gatoh and his lackeys have nothing to do with the future number 1 hero. —Bakugo said with such confidence that it seemed he could see the future.
"Heroes don't exist," Inari replied, going to his room.
—You be...! Take that back, you son of a bitch! —Bakugo exclaimed, standing up, but once again a sharp pain made him fall to the ground. —Fuck...!
"Where are you going, Inari?" her mother asked.
"To look at the ocean from my room," Inari replied.
—Tsk. I hate kids, —Bakugo muttered.
"You're the only kid here, Bakago," Sakura said, giving him a slap. "Listen to Kakashi-sensei and rest until tomorrow."
"Tsk," Bakugo muttered, biting his tongue.
The next morning, the four ninjas went to the forest to begin training under the instruction of Kakashi, who was on crutches. Bakugo's arms were bandaged, but he could move them with little pain.
"The training begins now. As you know, chakra is the elemental life energy that a ninja uses to create their jutsus. The source of all a ninja's power. This energy comes in two forms: physical energy, which resides in the body's cells, and spiritual energy, which is the original source of power that is increased with training and experience. That is, the body and the mind. Both, body and mind, must be used and combined to perform jutsus," Kakashi explained.
"We're already using charka with our techniques. I don't understand why you're explaining this to us, Kakashi," Sasuke said.
"Even if you can perform jutsus, you haven't mastered chakra yet. You've only just scratched the surface. You have to tap into physical and spiritual energy and combine them within yourself in the proper ratio. Each technique requires a different combination of these energies. So far, you've been improvising the ratios, hoping they'll be just right. Even if you produce a lot of chakra, without balance and control, it won't be worth anything. You'll lose energy, run out of chakra, and become an easy target. Which is what happened to you in the fight against Zabuza, Bakugo," Kakashi reasoned.
"Yeah, yeah. It's clear now, stop remembering it," Bakugo muttered. "And how do we control chakra, gray-haired?"
"By learning to control your chakra without thinking," Kakashi replied. "To achieve this, you must be prepared to put your life on the line."
"What do we have to do?" Sakura asked, swallowing hard because of her nerves.
"Climb a tree," Kakashi replied, pointing upwards.
"Huh?" the three genin said at once.
"You heard right. Climb a tree. But there's only one rule. You have to do it without hands," Kakashi said.
Bakugo's curiosity grew ever more intense. He loved climbing, but he'd never done it without using his hands. How can you climb a tree without hands? That's what the ash-blond boy wondered. Which was exactly what Sakura asked.
"How can you climb a tree without using your hands?" Sakura asked.
"Watch," Kakashi replied. Then, he used a chakra seal to focus his chakra on his feet and walked toward the nearest tree.
Kakashi walked up the tree as if it were on the ground, defying the laws of gravity. The three genin stared at Kakashi in admiration, as if he were performing magic. Bakugo was the most excited.
—Heh. —Bakugo smiled.
"Focus your chakra on the soles of your feet and use it to connect to the tree. The chakra will act as a kind of glue," explained Kakashi, who had lain face down on a branch. "This is one way to apply the power of chakra."
—Wait a minute... That trick is all very well, but... what good will it do us against Zabuza?—asked Sakura.
"It's the only way to confront him, Sakura," Kakashi replied. "Facing ninjas as powerful as Zabuza is the goal of this training. First, you learn to draw the precise amount of chakra from a precise point on your body. This form of climbing requires a subtle combination of physical and mental energy. And the soles of your feet are the most difficult places to concentrate chakra. If you can master this, you will be able to master any jutsu. Second, you learn to maintain your chakra levels. Maintaining your chakra level should become second nature and effortless."
"I see..." Sakura said, but without understanding the tone.
"Now you must apply the power of your chakra during your training." Kakashi then threw three kunai, one at each student's feet. "Use the kunai to mark the tree at the highest point you can climb without using your hands. Then, try to climb past that mark again and again. At first, you'll have to run toward the tree so that your momentum will carry you as high as possible. Until you master it and do it like me. Ready?"
—Ha. This is nothing for the one who is going to become the number 1 hero. —Bakugo replied.
"I thought you wanted to be the Fifth Hokage, Bakugo," Kakashi said, his head aching.
"I'll be both!" Bakugo replied.
—That's it. Get your thoughts straight, Bakago! —said Sakura, punching him again.
—Stop hitting me for no reason, macrofront!—Bakugo shouted.
—I will when you stop being such a baka (idiot), Bakago!—replied Sakura.
Sasuke, ignoring his companions' argument, was the first to attempt to climb the tree. He ran to gain momentum. It seemed as though he was going to make it, but after making it a third of the way up, a crack appeared in the tree, preventing him from continuing.
—This is harder than I thought. Your balance has to be perfect. If you concentrate too much chakra, your feet cause a crack in the bark that makes you bounce. But if you don't concentrate enough chakra... —Sasuke thought, while watching Bakugo's attempt. Bakugo did much worse than Sasuke, barely managing to climb the tree and falling backward. —... you fall like Bakugo did when he couldn't connect to the tree. —
—Shit... All I know about using chakra is that with my emotions I manage to concentrate it in my hands, and when I accumulate sweat I manage to secrete nitroglycerin. But I've never tried to concentrate the chakra in another part of my body. Fuck, at this rate I'm going to fall behind Sasuke and the big forehead. —Bakugo thought, getting up from the floor. —And the worst of all is that my arms still hurt. Not so much that I can't move, but enough that falls hurt me quite a bit. —
—Just what I thought would happen to Sasuke and Bakugo. —Kakashi thought.
"Ha. This is so easy!" Sakura exclaimed from high in her tree. "Hahahaha."
"No way! The big-foreheaded girl got it the first time!" Bakugo said, impressed, but annoyed that he wasn't the one who did it.
"Wow, it seems Sakura is the one who has the best chakra skills. Well done, Sakura," Kakashi congratulated her.
—Tsk. —Bakugo muttered.
"Bah," Sasuke said, rolling his eyes.
"I thought it would impress Sasuke, but now he's mad at me," Sakura thought sadly.
—Not only does Sakura have a wealth of theoretical knowledge about chakra, but she can also control and maintain it. Therefore, Sakura is the closest to becoming Hokage one day, unlike a certain someone…—Kakashi said, looking at Bakugo. The ash-blond narrowed his eyes, wanting to kill his sensei.—...And as for the great Uchiha clan, it might not be so great after all. —
—SHUT UP KAKASHI-SENSEI! —Sakura shouted. —You're going to make Sasuke hate me even more!—
Sasuke and Bakugo looked at each other for a moment.
"I'm not going to let Bakugo surpass me," Sasuke thought with determination.
"I'm not going to let Sasuke surpass me," Bakugo thought, clenching his fists.
"Good, I think they're motivated now. It's clear that Bakugo and Sasuke have a lot more chakra inside them than Sakura. If they can master their chakra, they'll become exceptional ninjas," Kakashi thought.
Five hours later, the two boys still hadn't made much progress. Bakugo had managed to climb to 20% of the top, and Sasuke to almost 40%. Sakura, after climbing to the top of the tree about twenty times, was completely exhausted.
"Fuck, this is fucking shit!" Bakugo complained, enduring the pain he felt in his arms and head from the falls.
"How predictable, he always complains when things don't go his way," Sakura thought. Seeing Bakugo approach her, she was surprised. "Huh?"
—Forehead, tell me your trick to climb. —Bakugo asked.
"Is that how you ask for things?" Sakura said indignantly. "If you just tell me, maybe I'll think about it."
"Grrrrr," Bakugo growled, trying to contain his rage. "Please, Macro Front, show me the fucking trick!"
"No, no, no," Sakura said, shaking her head with her index finger. "No rude words or stupid nicknames. Just ask like a normal person would."
"Bah, it's not worth it." Bakugo turned around, but when he saw Sasuke climbing more than twice his personal best, he turned back. "Sakura... would you be so kind as to show me your climbing trick?" Bakugo asked, his body trembling from how embarrassed he felt.
"Wow, I didn't expect you to be so desperate, Bakugo," Sakura replied with a smile.
"You'll be...!" Bakugo said, clenching his fist.
"Calm down, Bakugo. I was joking," Sakura said. "I wouldn't call it a trick, but the best way to control your chakra is to be relaxed. Chakra requires spiritual energy, so if you're anxious and restless, it won't help you. You have to relax and focus completely on the tree. That way, you'll get a steady flow of chakra into the sole of your foot."
After that advice, Kakashi went to the builder's house to rest and asked Sakura to go protect Tazuna on the bridge he was building. So Sasuke and Bakugo were left alone, competing to see who could reach the goal first.
"Think about what the big-foreheaded girl said. Relax, focus on the tree. I'm too impetuous, I have to be cold like that bastard Half-Half." Bakugo thought, taking a deep breath to achieve it.
Sakura's advice proved effective, as she tried again to reach Sasuke's mark. As he fell from the tree, Sasuke approached him.
"Hey, Bakugo," Sasuke said, scratching his neck.
—Huh? What the hell do you want, Sasuke?—Bakugo asked, annoyed.
—Well... the thing is... —Sasuke said, not knowing how to say it.
"Get to the point, dammit," Bakugo said, sitting up.
"Hey, you asked Sakura for advice when she was here. What did she say?" Sasuke asked, turning his head away so she wouldn't see how embarrassed he felt for asking for help.
"Ha. Don't even think for a moment that I'm going to help you surpass me. I'll only tell you from the top of the tree, and only if you beg me to." Bakugo replied with a smirk.
"Damn..." Sasuke said, angry with himself for having thought of asking for help.