DISCLAIMER: I do not own Dragon Ball Z or Harry Potter.

Albus Percival Wulfric Brian Dumbledore stood over his desk, glaring at a list of reports about the Ministry's distrust of his and Harry's statements. It had occurred to him that with Voldemort back the school was at great risk, and also that the Ministry could exploit any empty staffing positions for their own use. In short, Dumbledore needed to find a new Defence Against the Dark Arts Teacher quickly. Luckily, something had caught his attention.

Son Goku sat in the top fork of a tree, nursing a nest of small birds whose mother was injured.
"Don't worry, little guys. Your mother's gonna be better soon!"
Goku flinched as a man suddenly appeared not far away, sitting in the fork of the tree on the other side of the bird nest from Goku. The man was elderly, with a long white beard and strange clothes. Goku looked at the man with curiosity, blinking in confusion. Seeing this, the old man smiled.
"Son Goku, am I right? My name is Albus Dumbledore. I am the headmaster of a school, and I need your help."
Goku blinked again, still confused, but answered, "I don't know how I can help. And these birds need me right now."
Dumbledore raised his wand and waved it over the mother bird, healing her injuries. She jumped up and puffed her feathers, triumphantly reassuming her position with her offspring. Goku nodded, and drifted away from the tree several feet.
"So… What kind of help do ya need?"

Goku sat in Dumbledore's office, admiring the instruments and various mystical objects in the office. Dumbledore took a sip from his drink, and explained, "We have a vacant teacher's position here, and we need you to fill it."
Goku looked blankly at Dumbledore. "You want me to fill your teacher's what?"
Dumbledore smiled. "We need you to teach Defence against the Dark Arts to students. There is a dangerous wizard in the world who is a threat to us, and the students need to be able to defend themselves."

Goku nodded, understanding. "You want me to teach kids how to defend themselves against bad guys?"
Dumbledore nodded, amused by the blunt way Goku thought about the situation. Some would call it stupid, but Dumbledore saw it for its practicality. Goku didn't care that Voldemort was a dark lord, the owner of a name that made idiotic wizards urinate in their pants, or the wielder of dark magic. To Goku, Voldemort was just a bad guy. That meant Goku would never fear the overrated name, never be fooled by Voldemort's words or reputation. To Goku, Voldemort was just another enemy.

Dumbledore leaned forward. "So, what do you say?"
Goku blinked. "About what?"
Before Dumbledore started to elaborate, Goku nodded. "Oh, the teaching thing. Yeah sure. I'll do it."