Miyamoto Usagi is the titular character (inspired by Miyamoto Musashi) and an anthropomorphic rabbit (Usagi is Japanese for rabbit) and a ronin now walking the musha shugyo (the warrior's journey). 
He is a highly skilled swordsman and one of the best in the land. Given that his father is referred to as "Magistrate Miyamoto", it is assumed that Miyamoto is his family name, which follows traditional Japanese naming conventions with the family name appearing first.
Usagi was born the only son of a village headman. His two childhood playmates were Kenichi (with whom Usagi would have a less-than-friendly rivalry his whole life) and Mariko (one of the reasons for the boys' rivalry). Eventually the trio went their separate ways when the boys were sent to be trained as samurai in the Dogora school of bujitsu (arts of war). However, on the trip there, the boys witnessed a confrontation where a gang of arrogant Dogora students attacked a lone traveler named Katsuichi, who had left the school years ago, dissatisfied with the poor calibre of the students. In spite of their numerical advantage, the gang was quickly defeated by the sensei's unusual, but definitely effective, technique. Although Kenichi was unimpressed by the display, Usagi pursued the departing sensei to petition him to become his student. Katsuichi initially refused, but relented when Usagi stood outside his home day and night through unpleasant weather long enough to convince the teacher of his determination.
For years, Usagi was the exclusive student of Katsuichi-sensei, and although he proved a mischievous pupil who got into various misadventures, he also excelled at his studies to become a formidable warrior. One of those misadventures involved a young Usagi stealing a dying soldier's wakizashi while walking through a battlefield with his teacher. Fraught with guilt over the theft, Usagi began seeing the now dead soldier in numerous situations, at one point having a nightmare that involved Katsuichi morphed into the soldier. Upon his return to the battlefield to return the sword to its rightful owner, Usagi was caught by adult samurai and accused of theft. Instead of summarily executing the young rabbit, they instead began the process of cutting his hand off. Before a blow could be struck, however, Usagi's future lord, Mifune, intervened on his behalf and upon sensing the rabbit's honorable nature, let Usagi go.
At the end of his training, Katsuichi brought Usagi to a fencing tournament hosted by the Dogora school. Usagi won the tournament, his final match being against his old "comrade" Kenichi (now the top student of the Dogora school), and earned his own daisho: the katana named Yagi no Eda ("Willow Branch") and the wakizashi named Aoyagi ("Young Willow"). The region's daimyo, Lord Mifune, was observing the contest and was impressed with Usagi's skill enough to offer him a position as a retainer. Before leaving to enter Mifune's service, Usagi returned to his village for a final farewell, where he found Kenichi had been staying at an inn in drunken despair, having sworn to leave the school due to his failure to win the tournament, but too ashamed to return home. Together they returned to their village to free it from brigands that were threatening it. Kenichi decided to stay (and would eventually become headman upon Usagi's father's death), which comforted Mariko somewhat against the loss of her other dearest friend. Kenichi and Mariko later married; however, before leaving the village, Usagi and Mariko had a romantic encounter that resulted in a son, Jotaro, whose parentage was hidden from Usagi for years.
In his term of service, Usagi rose to become a trusted personal bodyguard of his Lord and his family. That stable career was destroyed when a villainous rival lord, Lord Hikiji, sent ninja to assassinate the Mifune family. In the assault, Mifune's wife and son were murdered and Lord Mifune waged war on his rival in revenge. That war concluded at the battle of Adachigahara (sometimes referenced as Adachi Plain), where Mifune had the upper hand until Buichi Toda, one of his subordinate commanders, betrayed him and joined with Hikiji. Then Usagi's friend and immediate superior, Gunichi, fled the field upon seeing that the battle was lost. Lord Mifune was killed by an arrow barrage; Usagi performed his final duty, which was to escape with his lord's head to prevent the enemy from displaying it. As he fought free, he had his (so far) only personal confrontation with Hikiji, which left him with his distinctive arched forehead scar. Usagi escaped into the forest, buried Lord Mifune's head, and eluded pursuit by Hikiji's forces. (By saving Lord Mifune's head from desecration, Usagi felt he had atoned for the disgrace of losing the battle. Otherwise, he would have felt compelled to commit seppuku.) Usagi has since avenged his master's death upon both Toda and Gunichi, although Hikiji remains beyond his reach.
Now a ronin, Usagi traveled the backroads of the region making a living as a bodyguard for hire (yojimbo). In the course of his "warrior pilgrimage", he made deep friendships with many including the young Lord Noriyuki of the Geishu Clan and his valiant bodyguard, Ame Tomoe, the cynical rhino bounty-hunter Murakami Gennosuke, the brilliantly astute Inspector Ishida, the cat kunoichi (female ninja) Chizu, and the sly street-entertainer/petty thief, Kitsune.
Miyamato Usagi was voted by Empire magazine as one of the 50 Greatest Comic Book Characters.