Anime can't always be about the young kid with the destiny to save the world. As much fun as that sounds like, that kid also had to be trained by someone else who knew what they were doing, preparing them for the battles they would have to deal with in the future.
Over the years, anime has given a surprising amount of love to its teacher characters. Slice-of-life teacher series like Great Teacher Onizuka are considered classics of the era they came from. At the same time, there are some teacher characters who aren't there for comic relief—they're there for action. These characters teach their students by being an example of the lessons they're teaching, helping all their students reach their full potential.
10 Koro-Sensei Can Move At Mach 20 & Can Only Be Hurt By Special Weapons (assassination classroom)
Koro-sensei was originally the deadliest assassin on the planet. But a series of experiments transformed him into his new form... which is somehow even more dangerous. Now, he has the ability to move at speeds around Mach 20, and conventional weaponry doesn't hurt him even when it does hit him.
The only thing that can hurt him are weapons made out of special material that most people don't even have access to. Fortunately, Koro-sensei is more worried about paying bills and teaching students than he is causing anyone any real harm anymore.
9 Jiraiya Was One Of The Legendary Sannin (naruto)
When Naruto needed someone to take his training to the next level, he was fortunate enough to meet Jiraiya. Jiraiya was a knuckle-headed ninja just like Naruto, but his skill with ninjutsu was unparalleled. It was with Jiraiya that Naruto learned how to use the Rasengan, as well as tap into the power of Sage Mode, which comes from drawing natural chakra into one's body.
Jiraiya was a master ninja, talented both in combat as well as espionage, and someone who was even capable of fighting evenly with Pain, leader of the Akatsuki.
8 Izumi Curtis Trained A Teenager To Become A State Alchemist (fullmetal alchemist)
Izumi Curtis was responsible for training Edward and Alphonse Elric in their mastery of alchemy. She not only served as a replacement mother for them as they aged, but taught them everything they knew.
Considering Edward managed to become the youngest ever State Alchemist, that's pretty impressive. At the same time, she also taught him how to become a talented martial artist, meaning he's capable of defending himself both with alchemy and with hands.
7 Kakashi Hatake Is Master Of Hundreds Of Ninjutsu (naruto)
Kakashi trained all the members of Team 7, Naruto, Sasuke, and Sakura. A young prodigy, Kakashi was known for being a jonin with the harshest, most exacting standards. He's failed more students than he ever passed... but the students he passes go on to be fine ninja.
Kakashi was originally known as the Copy Ninja for having learned so many ninjutsu with the help of his Sharingan. He's also the creator of the Chidori, a lightning technique that only wielders of the Sharingan could use effectively.
6 All Might Is The Symbol Of Peace (my hero academia)
All Might is the Symbol of Peace for society, a title he got for introducing a new era of peace for humanity after decades of villains being allowed to run wild. His strength comes from the power of One For All, which grants him incredible strength, speed, and durability.
Despite going up against one of the most dangerous villains in All For One, All Might was able to not only put him down, but make the people believe in heroes while doing so. His powers no longer work since passing them to his successor, Midoriya Izuku, though, so he spends most of his days teaching the next generation at U.A. High.
5 Kisuke Urahara Pioneered New Methods To Attain Bankai (bleach)
Urahara isn't just a good teacher, he pretty much reinvented what it takes to become a top-class Shinigami. Ichigo learned in a matter of days what's supposed to take centuries to figure out, all thanks to Kisuke's unique methods of training. And even besides that, Urahara is one of the strongest Shinigami to have ever been part of Soul Society.
By the end, he's one of the only people capable of fighting Aizen, and even though he loses, he still proves himself stronger than any of the other Captains. Not merely a master swordsman and Kido specialist, his shikai provides him a variety of different abilities that keep him from even having to use his bankai usually.
4 Biscuit Krueger Has Some Dangerous Teaching Techniques (hunter x hunter)
Biscuit Krueger was another trainer that Gon and Killua ran into who helped them become better Hunters. On Greed Island, her strength and speed were without equal, which is how she was able to easily defeat Binolt, a Hunter who would have killed Gon and Killua if he'd confronted them alone.
She has a variety of Nen abilities, but most of the time she hides her true power to resemble a young girl. And while she's a good teacher, her training methods are questionable at best. To help Gon and Killua, she made them fight Binolt, a cold-blooded murderer, for ten days straight to see if they could improve enough to land a hit on him.
3 Piccolo Destroyed The Moon Training Gohan (Dragon Ball Z)
Piccolo was once the most dangerous threat to the entire planet. And then the Saiyans came, and that immediately readjusted what "dangerous" really meant. Still, while training Gohan, Piccolo had to face Gohan in his Giant Ape form, which made Gohan ten times as strong and crazy with rage.
The only way Piccolo was able to stop him was by blowing up the moon itself, a feat that should be impossible but is normal in the Dragon Ball Z universe. From there, Piccolo continued to grow in power and has trained Gohan several times since then to face threats to their home.
2 Genkai Was A Legendary Martial Arts Teacher (YuYu Hakusho)
Years before Yusuke was born, Genkai was a martial artist alongside her partner Toguro, and they both ran a dojo together. But a demon killed all of their students, leading both Toguro and Genkai to get revenge by entering and winning the Dark Tournament. Toguro would choose to become a demon himself, while Genkai, disappointed with her partner, secluded herself.
While her strength should have waned, she didn't become much weaker at all. Instead, despite all their training leading up to the Dark Tournament, Genkai was the strongest member of Team Urameshi. She even had enough power to give most of her energy to Yusuke and still continue to fight on the team.
1 Whis Trained Beerus To Be A God Of Destruction (dragon ball)
Despite Beerus being a God of Destruction and having the power to deal with Goku not only at Super Saiyan God but Super Saiyan God Super Saiyan, he's not the strongest fighter. It seems that way, until Beerus admits that it was Whis who taught him everything he knows.
As an Angel, Whis is effectively Beerus' handler, and he's usually the only person that feels up to training Goku and Vegeta. As a result, Goku and Vegeta have both started fighting more like angels, with Goku even developing the powerful Ultra Instinct ability. Still, Whis' one drawback is that he can't actually interfere in the affairs of mortals, whether he wants to or not.