DISCLAIMER: I do not own Akira Toriyama's Dragon Ball or JK Rowling's Harry Potter.

Goku stood, shirtless, in front of the sink in the bathroom, wringing the lake water out of his hair and clothes. He had already wringed the water out of his pants and put them back on. As he squeezed the water from his hair he felt something rough, and plucked it from his hair, jumping when he looked at it and saw that it was a small tentacle. He had no idea what lived in that lake, so he decided to simply disregard the tentacle and drop it in the sink. As he started to put his shirt back on, the door opened behind him and he turned to see Hermione Granger and two other girls.

Hermione stepped forward, glaring at Goku and saying, "You do realise this is the girls' bathroom, right?!"
Goku blinked. "You mean you girls own it? Do I have to pay rent or something?"
Hermione sighed in annoyance. "I mean that you shouldn't be here!"
As Hermione started to speak further, one of the other girls walked over to Goku and looked into the sink Goku was next to.
"Ah! A Grindylow tentacle! That's an interesting souvenir!"
Goku turned to regard the tentacle Luna was admiring, and smiled naively. "Yeah… I just found it stuck in my hair!"

Hermione rolled her eyes. "Well that's what happens when you jump in the black lake; a bloody Grindylow attacks you! It isn't rocket science! You're lucky it wasn't the giant squid!"
As Hermione berated Goku, the door opened again to admit Parvati and Padma Patil, who both attempted to appear as if they weren't looking but were very obviously admiring the shirtless Goku's physique. Goku waved to the two girls, aware that they were looking at him but oblivious to the reason why. Parvati giggled as she stared, and Hermione rolled her eyes, placing herself in front of Goku to block Parvati's view. Parvati giggled again as she observed Hermione's short temper, and the innocent look Goku wore simply elicited more intense giggling as Parvati headed out of the bathroom.

As soon as Parvati and Padma left, Hermione turned back to see that Ginny was now admiring Goku's physique, and by contrast Luna was still admiring the Grindylow tentacle. Hermione shook her head in annoyance at Ginny, and simply said, "Goku, this is the girls' bathroom. That means only girls should be in here."
Goku nodded innocently, and Hermione held up her hands in front of him, gesturing as if she were talking with her hands.
"So you need to find another bathroom to get changed in. There are ones for staff. Ask Professor McGonagall."
As she finished, Goku nodded and smiled. "Okay. Sorry…"
Hermione jumped as the door opened again, admitting several girls from Slytherin, and Hermione turned to face Goku, and was surprised to see he had already disappeared. She felt relieved until she saw that his damp shirt was still in the floor. She quickly picked it up and screwed it into a ball so that the Slytherin girls wouldn't recognise Goku's clothes.

Goku appeared in Dumbledore's office, having used Dumbledore's Ki signature to teleport to him with Instantaneous Movement. He turned to see an amused Dumbledore, and suddenly realised that he'd forgotten to put his shirt back on.
"Gee… I left my shirt in the girls' bathroom…"
Dumbledore chuckled at this ridiculous-sounding statement, and said, "It's best you don't say that to anyone else. To say they might misunderstand would be an understatement."
As Goku got up to leave, Dumbledore called out at his back, "I should warn you that Umbridge is about to start inspecting the staff. If she finds any excuse to remove you she will do so."
Goku paused at the doorway. "So how do I stop her doing that?"

Hermione knocked on the door to Goku's office, and opened it when a voice she didn't recognise told her to enter. She stepped in to see Goku meditating on the floor, and a large green creature reminiscent of a man hovered in the same meditative state directly across from him. She gasped as she beheld the creature, but it turned to her and said in the deep voice that had told her to enter, "I know I'm green; stop staring."

Hermione flinched at this, and stammered, "S…so… sorry, sir. Forgive me, but… who are you?"
The green man glared at her, and then suddenly put his feet on the floor, standing up and towering over the girl at a height greater than she had seen save for Hagrid. The green man stepped over to her, and folded his arms.
"What are you here for?"
Hermione held up Goku's shirt, now not only dried but folded and cleaned, and the green man took it gently from her, tossing it onto the nearby desk.
"Please tell me he didn't leave it at your place."
Hermione widened her eyes in horror. "Of course not! He… dropped it."
The green creature nodded. "Okay. Thank you."

Hermione waited for a moment, but the green man simply sat back down, paying her no mind. As he sat down Goku stood up, walking over to Hermione and thanking her with a more grateful tone of voice. Hermione and Goku exchanged a look of understanding, and Hermione asked, "Who is your friend?"
Goku nodded. "Hermione, this is Piccolo. He'll be helping out a little bit. That's basically all there is to know."
Goku smiled in a goofy way, and Hermione knew there was more, but didn't press the matter.
"Okay. Good luck professor."

As Hermione left, Piccolo stood up. "She seems decent enough. I understand why you want to help these people. It seems out of character for you to stop training for a whole year though."
Goku smiled that same goofy smile. "Well actually this is training in a way. Learning about their magic might help me become a better fighter. And besides, the old man said he'd pay Chichi all she wants. Once I finish here I can go and train with Vegeta again. Plus it's all for a good cause. Where's the downside?"
Piccolo cracked a cramp out of his neck, and held up a flask, taking a sip of water.
"From what I understand this Voldemort guy actually might be that downside. He does seem to have weapons that can hurt even us. Even you, Goku. There are things called Dementors, for example. Do not let them surround you."

Goku laughed. "Never heard of them… but the old man said something about flying things in robes that I have to avoid. Whatever. It's not like I'm scared of danger."
Piccolo closed his eyes and took another sip of water. "Whatever. Was there anything else you wanted from me?"
Goku huffed with satisfaction. "You bet."

Goku sat on his desk, waiting for his class. It just so happened that it was Harry and Hermione's class. As the class entered Umbridge walked in behind them, holding a clipboard. She took one look at Goku and started writing on the clipboard right away. Goku blinked with confusion, wondering what he could have done that was worth writing about when he hadn't even stood up yet. As the last student was seated, Goku stood up, and Umbridge again started writing on her clipboard. Goku still couldn't imagine what she could be writing about him when he hadn't even done anything yet, but he diverted his attention to the students.
"Morning, everyone. I have a few books to pass out today. I ain't gonna be teaching from them, but they have all sorts of useful information in them."

Goku started to head for the pile of books Piccolo had conjured for him, but he saw that Umbridge was once again writing on her clipboard.
What in the world is she writing about? I haven't even done anything yet and the class only started a few seconds ago!

Goku shook himself to get Umbridge out of his mind, and turned to get the books, lifting them telekinetically and moving them to every desk in the room.
"Read it when you want and then just return it so other people can read it."

As Goku spoke, Hermione's hand shot up, and Goku said, "Don't raise yer hand, just talk."
Hermione stood up, holding up the book having already read through quite a bit of it.
"This book says that the best way to overcome any adversary is to crush it. It has calculations for how much each type of enemy can withstand in terms of…"
Hermione read the book and continued. "In terms of pressure, heat, cold, oxygen deprivation, piercing wounds, explosive power, wattage, and several other things. I think…"

Hermione had been planning to refer to the book as barbaric and say whoever wrote it believed in destroying everything in front of them, but instead she turned to regard Umbridge who stood at the back with her quill poised, obviously planning to write whatever Hermione said if it was negative, which she knew her tone suggested it would be.
Realising what Umbridge's intent was, Hermione finished, "I think whoever wrote this has an excellent grasp of both magic and science. Not only does it have weaknesses for almost any foe listed, but even says how much force is required to beat it. A full encyclopaedia most informative."

Hermione sat back down, briefly turning to regard Umbridge, who had turned a light shade of red and was gripping her quill as if she wanted to stab Hermione with it.
As Hermione turned away from Umbridge, Piccolo's voice said in her head, "Well saved, kid. I don't appreciate being called barbaric but at least Goku doesn't get fired. Not for that anyway…"

Goku then stepped over to the door, opening it to admit Piccolo, who entered the room and glared at the students.
"I'm an assistant for Professor Son. You can call me Professor Piccolo or Piccolo-Sensei. Anyone who calls me green guy or pointy ears or anything like that is going on detention where they'll be making all Professor Son's meals for a week."
As Piccolo spoke, Malfoy was laughing at him, along with Crabbe and Goyle. Piccolo turned and glared at the three bullies, cracking his knuckles by clenching his fists.
"Is something funny?"
Malfoy shook his head sheepishly, and Piccolo nodded. "That's good. Keep it that way."
After a few seconds of silence, Ron asked Piccolo what the equations in the book meant, and Piccolo answered him, while Malfoy leaned over to Goyle and whispered something.
Piccolo suddenly stopped answering Ron's question and turned to face Malfoy again.
"I may be a green freak, as you so eloquently put it, but you're delusional if you think I'll grovel because you call your father. Detention for all three of you. Each of you spends a week making all of Goku's meals, and that includes making more when he asks for it too."

Umbridge started to write something else on the clipboard, but Piccolo suddenly extended his left arm to a length of several metres to reach across the room and snatch the clipboard from her. Piccolo read it and looked up at her with a sarcastic look.
"You noted that Goku sits on his desk, and that he stands up slowly?"
Umbridge stood up as tall as she could and started to speak, but Piccolo interrupted her, reading further.
"You mentioned that he says "ain't" instead of "isn't", and that he has a green teaching assistant who may or may not be above maximum height for a teacher?"
Umbridge ground her teeth. "It is well within my right as High Inquisitor to note down the many things wrong with this teacher and his methods. Under the law, as enacted by the Ministry…"
Piccolo read further, interrupting her to say, "You noted that his teaching assistant was taller than him and that the books use what you call… "Muggle numbers"… I also made general equations for how much force most attack spells have behind them along with instructions on how to use them and how they work, so I think you'll find my work is better than anything you'll ever write."

Umbridge stamped her foot. "How dare you?! I should warn you… under the law, as a creature of near-human intelligence…"
Piccolo read her clipboard further, and interrupted her again to say, "That's funny coming from someone who dismisses numbers and math because a race she doesn't like invented them."
Umbridge clenched her fists and ground her teeth. "I assure you… the Ministry will have something to say about you threatening violence against the senior undersecretary and denigrating the Ministry itself as inferior to Muggles! I will not stand for such lies!"

Piccolo turned to regard Goku, who for his part looked uncertain what to make of Umbridge's outburst. After a moment of silence, Piccolo said to Goku, "I'm starting to miss the good old days when I just used to blow people like her up."

Harry heard Ron snicker beside him, and he felt inclined to do the same thing. He turned just in time to watch Umbridge storm out, eliciting a sigh of relief from several students.
Piccolo shrugged, crushing Umbridge's clipboard in his hands and throwing it out an open window. He then turned to the students. "All right, first off turn to the back of the book. That's where you'll find information on Voldemort himself and the creatures he has in his army. You'll find instructions on the best way to beat the monsters that work for him and to deal with the spells his servants use. There's a biography of his own past and some equations and information on him and his top soldiers too. He's probably gotten stronger since though, so don't rely on the equations too much or you'll probably just underestimate him."

After a few seconds to let that sink in, Piccolo clapped his hands, shocking the stunned students back to reality.
"So let's cut the crap and get started."