Some TMNT stuff was always meant for grownups.
Miyamoto Usagi is a character created by Stan Sakai. He first appeared in Albedo magazine, but was popular enough to start his own series, Usagi Yojimbo. On occasion, Usagi has appeared in cross-overs with the Teenage Mutant Ninja Turtles.
Miyamoto Usagi was born the only son of a village headman. In his younger years, Usagi and his childhood friend Kenichi were sent to be trained in the arts of war at the Dogora school, but on their way Usagi witnessed a band of arrogant Dogora students attacking a lone swordsmaster by the name of Katsuichi, who defeated his challengers with uncanny ease. Awed, Usagi chose to train with Katsuichi instead and with much determination became his exclusive and dedicated student. During one of his adventures, he first met the man who would become his master in young adulthood, Lord Mifune, who was impressed by Usagi's character and skill and eventually took him as a retainer.
Usagi served his lord well and faithfully, attaining the rank of personal bodyguard to Lord Mifune, until Lord Hijiki, an ambitious and evil daimyo, had Mifune's wife and son assassinated. Mifune moved to war against Hijiki, but in the midst of the fray one of Mifune's allies turned traitor, turning the battle into a slaughter. Mifune fell, but Usagi survived and escaped the battlefield with his lord's head to avoid having it displayed as a victory trophy by Hijiki. After burying the head, Usagi took to wandering the country as a ronin, perfecting his fighting skills as a shugoysha and following the warrior's path of learning, occasionally serving as a bodyguard (or Yojimbo), or helping his friend Gen as a bounty hunter. Usagi's sense of samurai honor and pride often led to him helping those in need and sticking his nose in other people's business which has gotten him in trouble on more than a few occasions.
One or more of the Teenage Mutant Ninja Turtles have appeared in the Usagi Yojimbo stories listed below. All stories were written by Stan Sakai unless otherwise noted.
- Usagi is based on the legendary samurai, Miyamoto Musashi. Usagi's name, which is Japanese for rabbit, is also a vowel pun on Musashi's name, containing the same vowels but different consonants.
- There was a banner with an image of Usagi on the set of Teenage Mutant Ninja Turtles III 
- Usagi stars in two of his own video games
- Samurai Warrior: The Battles of Usagi Yojimbo
- Usagi Yojimbo: Way of the Ronin