While those chosen as Einherjar usually return to their prime, he appears as an old man, even older than when he died in the duel at Ganryu Island. Because he was significantly older than Lu Bu or Adam from the first two rounds, and because he was chosen over Miyamoto Musashi—the victor of the Ganryu duel—his participation was met with skepticism from humanity's side, including Musashi's own associates. However, in Kojiro's case, he continued to pursue the way of the sword and evolve the Ganryu style even after death. Thus, his current aged self is his true prime, a fact he asserts with absolute confidence. His swordsmanship is so extraordinary that he can effortlessly catch a jar of milk falling from a desk, returning every drop of spilled liquid back into the jar with his blade. His sense of taste is somewhat unusual; when he claimed Brunhilde's Salmiakki pie was delicious, he received looks of pure disgust from Geir and his partner Hrist. According to Hrist, Salmiakki pie is essentially a "scent-cell destroying bomb." However, while he ate that under pressure, he apparently couldn't handle Isami Kondo's disastrous cooking when threatened by Okita Souji. Naturally, his fighting style is swordsmanship, wielding the Bizen Osafune Nagamitsu (over three shaku long), also known as the "Laundry Pole," which was Volund-forged by the second Valkyrie sister, Hrist. Furthermore, by crossing blades thousands of times in his mind with countless swordsmen, he possesses the special ability "Thousand Image Defense," which allows him to simulate an opponent's strength perfectly in his brain and perform the ultimate foresight. Because he fought his past victors repeatedly in his mind and eventually surpassed them, he is hailed as the "History's Strongest Loser." Even Musashi, upon seeing him enter the arena, declared him "Peerless Under Heaven."

The swordsman known for the duel at Ganryu Island against Miyamoto Musashi—but he is not that man himself. Normally, the Assassin summoned in the Fuyuki Holy Grail War must be one of the group of killers known as Hassan-i-Sabbah. However, because this summoning was a result of a Servant summoning a Servant—an unprecedented violation of the rules—a fictional Heroic Spirit other than Hassan was called forth. This explains his dignified swordsman appearance and combat style, which are ill-suited for an assassin. Although he introduces himself as Kojiro Sasaki, in the world of the Fate series, while a swordsman by that name may have existed, he only contributed the name to the legend. The legend of Kojiro Sasaki is said to be a composite of anecdotes from several master swordsmen of that era. As mentioned in the overview, he is a unique existence: a nameless wraith of a swordsman who best fit the conditions to form the Heroic Spirit "Kojiro Sasaki." Thus, he openly reveals his true name upon meeting Saber. His own life was one lived entirely for the sword. Born in a farming village deep in the mountains, he was captivated as a child by the technique of an elderly swordsman. However, that old man only swung his sword once before the boy, never even allowing him to become a disciple. Inheriting the "Laundry Pole" left behind by the old man, he continued to swing his sword while living as a farmer. All for the sake of approaching the transcendent technique he saw that day. And for the sake of cutting down a swallow as it glides through the sky. Just for that. He perfected his ultimate technique, "Tsubame Gaeshi," just before his death. In other words, despite possessing such skill, he never fought a duel in his entire life. Therefore, his wish is a serious match against a strong opponent—especially a fellow swordsman. "How could I call myself a swordsman without testing how far my secret sword can reach?" Incidentally, parallel worlds apparently existed where a real, non-fictional Kojiro Sasaki lived, though residents of that world claim that Kojiro wasn't nearly this strong. On the other hand, there are worlds where the so-called Kojiro Sasaki exists as a historical figure who mastered the Ganryu style, learned Tsubame Gaeshi, and lost to Musashi at Ganryu Island, only to be later regarded as his greatest rival; thus, he is a character with a wide range of interpretations. He generally refers to himself as "Watashi," and occasionally "Sessha" or "Soregashi." He is a man of refined taste who loves the beauties of nature and wears an elegant jinbaori. While mild-mannered, he has an elusive, aloof personality. Possessing the mindset of "Clear Mirror, Still Water," he is never shaken by anything, not even his own death. Conversely, he is a warrior to the core and prefers fair and honorable combat. As mentioned, since he spent his life training in the mountains and never crossed blades with anyone, his wish for the Holy Grail War is the act of combat itself against powerful foes. Regarding women, he makes flirtatious remarks toward Saber and Rider, yet shows a surprising side of confusion when Yukika Saegusa becomes fond of him, not knowing how to handle it. He seems to enjoy teasing women who put on an act, like Caster or Martha. Fans often refer to the swallows he cut as "TSUBAME" due to their seemingly supernatural evasion skills, and Kojiro himself as "NOUMIN" (Farmer) as a meme. Since he was born in a mountain village, it might not be that strange... Although he is manifested in the Assassin class, because he is an irregular Servant summoned through a loophole, he has lost almost all class traits of an Assassin. He seems more suited for the Saber class. Perhaps it is because he possesses the exquisite killing skill to cut a swallow... though as stated, he has never actually killed a person. In fact, during the Seven Heroic Spirit Swordmasters, he argued against Saber Empyrean's grand theories, stating that the way of the sword is the way of murder; Heroic Spirits skilled with blades often have an aptitude for the Assassin class. According to Kinoko Nasu, in terms of pure swordsmanship alone, he is the greatest Servant in this Holy Grail War (Saber's strength comes from overall power, not just technique). He utilizes sharp, one-hit-kill strikes described as attacks where every parry is aimed at taking the head.