Also known as Ganryu (he later named his martial arts school after it), he was trained by famous swordsmen in the art of wielding the kodachi (shortsword) and later the katana. By 1610, he became a powerful warrior, capable of fending off up to three opponents with a tessen (metal fan), and was named a Kensei, or "Sword Saint", someone who has perfectly mastered the blade. His favourite weapon was a nodachi called Bizen Osafune Nagamitsu which was also referred to with the less-formidable name of Monohoshizao (Laundry Pole).
He also developed a signature powerful sword technique called Tsubame Gaeshi (Swallow Return), a high-speed swordslash that changed direction at the last second.
Having found fame, Kojiro was eventually challenged by his long-term rival Musashi, and was killed at Ganryujima in April 1612. It's often thought that Musashi may have cheated, however, there are many stories about what actually happened, and no one knows the full details.
Kojiro is usually portrayed as a Bishōnen and a lady-killer, though it is also rumored he was deaf in one ear. Widely considered Musashi's most powerful opponent, and the two are often cast in a Red Oni, Blue Oni relationship (with Kojiro usually cast as the Blue).
References to Kojiro appear in the following works:
- Yaiba features Kojiro as a revenant swordmaster resurrected by the Big Bad Onimaru. He wields his trusty sword Laundry Pole, which can grow longer on command.
- Shigurui: In Ushimata Gonzaemon's anecdote about the short sword master Toda Seigen, Kojiro is mentioned as having been Toda's disciple. Since Toda's style was based on using the short sword to defeat the long, it was an ideal challenge for Kojiro to hone his skill with the long blade in their sparring.
- Kojiro appears in Vagabond as a deaf-mute but incredibly skilled and overall good-natured swordsman. He's also very pretty, and very popular with women.
- In Pokémon, the original name of James was Kojiro, while Jessie's was Musashi.
- In Yu-Gi-Oh! 5D's, Mizoguchi uses a samurai themed deck. Some of his card's illustrations show Kojiro and Musashi's famous duel on them.
- In Is This a Zombie?, the swordswoman Seraphim uses the Swallow Return technique, and constantly announces it.
- In an episode of the first series of Yatterman is a parody of the Ganryuujima duel, with the Doronmbo Gang helping out a Gonkish caricature of Kojiro (by supplying him with a rope to retrieve the scabbard after throwing it away and a tricked-out sword to hypnotize Musashi). Ultimately he and Musashi make peace and decide to train together.
- One member of the Bakka Hakken from Gamaran Shura named Shiba Hien is seemingly an Expy of Koujiro, being a young, good-looking and extremely talented swordsman with a swallow motif in his name and technique brandishing a nodachi whose name, (Monohoshizao Masamune) refers the derogatory nickname Musashi gave to his sword Bizen Osafune. He can also perform a technique similar to Tsubamegaeshi thankst to the handle of his sword.
- The version of Kojiro shown in Record Of Ragnarok is posthumous, but brought over to fight in what would be his prime if not for the duel with Musashi— an Old Master who's learned from all of his previous failures and opponents, gaining the ability to predict his opponent's next moves and adopt the styles of his previous adversaries into his own fighting style. He is also depicted as grateful for every battle he got to take place in, having a genuine love for swordsmanship and life in general. Because of this, he's introduced as "History's Greatest Loser".
- In the Altered Destinies multiverse (specifically in the stories written by Jim Bader), Sasaki is an Immortal who quickened after his famous duel then went on to create the Hiten Mitsurugi-ryū.
- In Hiroshi Inagaki's Samurai Trilogy, an adaptation of the Yoshikawa novel, Kojiro is portrayed far more positively than he was in the book. He appears as a starry-eyed young fop who is nevertheless an honorable warrior and a Master Swordsman. Kojiro still seeks fame and fortune through his swordsmanship, but is much kinder and less ruthless about it than he was in the novel. He doesn't even really harbor any animosity toward Musashi, instead seeing him as a Worthy Opponent.
- Tomu Uchida's five-part Miyamoto Musashi series, also called the Zen and Sword series, is a much closer adaptation of the Yoshikawa novel, featuring a young Ken Takakura as Kojiro. As in the novel, Kojiro is depicted as an arrogant Jerkass who is nevertheless a very skilled swordsman. However, he does not rape Akemi or try to sabotage Musashi during the Ichi-joji duel like he does in the book.
- Musashi by Eiji Yoshikawa. Kojiro is portrayed as an arrogant, sadistic, silver-tongued fop to contrast with Musashi, who is portrayed as a humble-but-spiritual vagrant. Musashi seeks enlightenment through the Way of the Samurai, while Kojiro seeks fame and fortune. Ultimately though, both men commit deeply cruel acts in their younger years, but eventually manage to improve their conduct.
- In Samurai Warriors Kojiro is introduced as a ghastly yet beautiful, narcissistic assassin very skilled with his Nodachi and extra-dimensional blades. He's Musashi's rival, and seeks to give him a ''beautiful end''.
- Appears in Onimusha Blade Warriors as an aloof, cold warrior very skilled with his blade. His rivalry with Musashi is also toned down.
- Fate/stay night has the "Assassin" who's based on Kojiro Sasaki and even uses the Tsubame Gaeshi and a nodachi. However, this version of Sasaki Kojiro goes by the theory that the man is a fictional rival created by Musashi. The servant known as Assassin is just a nameless man who perfected his own Tsubame Gaeshi technique and was just credited by this name since Heroic Spirits are in large part powered by their own fame and Kojiro became very famous despite not actually existing. That portrayal is played by Shin'ichirou Miki, who also voices the Pokemon character (James/Kojiro) mentioned above.
- Brave Fencer Musashi has Kojiro as Musashi's rival.
- Gandrake in Musashi Samurai Legend is based on Kojiro. He is actually him.
- Ill Swordmaster and Iaijutsu Practitioner Ukyo Tachibana is based on Kojiro. He even has the Tsubame Gaeshi attack, though his can only be done in midair.
- Pokémon references Kojiro with the move Aerial Ace. In Japan, it is named "Swallow Return" after his signature move. Most Pokémon can learn the move, despite it being Flying type (Just for Pun, due to its name) though it may be due to the fact that most of the ones that can learn it have claws.
- Final Fantasy VII's Sephiroth is based on Kojiro, having a beautiful and aesthetic appearance and fighting with an outrageously long nodachi. His rival Cloud uses a clumsy-looking oar-shaped sword, referencing Miyamoto Musashi.
- Rondoline 'Rody' E. Effenberg of Tales of Phantasia (in both the PSP version and the Narikiri Dungeon X remake) has her artes themed and named directly after the Tsubame Gaeshi, including one version with an Alternate Character Reading (making the first portion mean "Chain Wave Woman" instead of actually meaning "Swallow").