This showed up on my radar in 2021 as a well reviewed adult Anime film that I hadn't seen - from 2007. A little surprising.
Our two main characters are "Nanashi" or "No Name," a highly skilled ronin who has sworn never to unsheath his sword again, and a boy named Kotaro who is being pursued by a group of Chinese warriors. And Kotaro's dog Tobimaru. No Name and Kotaro meet when they both take refuge in an abandoned temple, and Kotaro hires No Name to get him to a distant temple. The most important remaining character is "Luo-Lang," a blond foreign master swordsman working for the Chinese. He's bored with Japan because there hasn't been a single worthy opponent.
I found the movie structure very conventional: Kotaro and No Name don't like each other, but grudgingly co-operate. No Name slowly becomes a father figure. It's clear from ten minutes in that the final confrontation will be between No Name (who will draw his sword) and Luo-Lang ... you probably guessed that just from my description. There are sword fights. There are many, many swordfights, and litres, no, swimming pools of blood are spilled - this isn't a kids movie. Although that's really only because of the blood: the logic is almost at the level of Kids' Anime. I'm going against the critics here, but I was distinctly underwhelmed by this film. If you want a movie about a kid and a grumpy mentor, see the far better, less violent, and far more beautiful "The Boy and the Beast" (even if you're an adult).