Basil, whose full name is Basilicum, is a young Italian boy with dirty blond hair and blue eyes, often with his fringe covering his right eye. He is a member of the CEDEF, the External Advisor organization of the Vongola Family, and serves directly as the apprentice of Iemitsu Sawada, Tsuna's father.

His personality is defined by his politeness, kindness, and a strong sense of loyalty to the Vongola, particularly to his master, Iemitsu, and the Sawada family. He is a strong-willed fighter who dedicates himself fully to his missions. A notable quirk resulting from his training under Iemitsu is his use of archaic, outdated Japanese, equivalent to using words like "thou" and "thee" in English. This extends to a habit of addressing others with the honorific "-dono". This has also led to him having some misguided ideas about modern Japanese culture, such as believing laundry should be done with a washboard, which he accepts without question due to his respect for his teacher.

Basil's primary motivation stems from his deep loyalty and sense of duty. He is eager to prove himself and assist the Vongola, especially Tsuna, whom he quickly comes to respect and support. He is first introduced during the Varia Arc, carrying the Half Vongola Rings to deliver to Tsuna. His arrival inadvertently leads the Varia assassin Squalo to Tsuna and his friends, which sets the arc's conflict in motion. After being defeated by Squalo, Basil plays a crucial role by helping Reborn train Tsuna for the upcoming Ring Battles, serving as a powerful sparring partner who forces Tsuna to grow stronger. Following the Vongola's victory, Basil gives his remaining Dying Will Pills to Tsuna before returning to Italy.

Basil's role in the story continues into the Future Arc, where he is transported ten years into the future. Upon arrival, he finds a Box Weapon and the Helper's Book, a manual that teaches him how to fight in the future, and makes his way to the Vongola base in Japan. There, he uses his Rain Dolphin Box Weapon to help calm Tsuna's berserk Vongola Box. He later participates in the final battle against the Millefiore Family. In the subsequent Curse of the Rainbow Arc, he continues to fight alongside Tsuna and Enma as part of a commando unit, battling the Vindice.

His most significant relationship is with his master, Iemitsu Sawada, who gave him his Dying Will Pills and taught him how to fight. Through his mission and training, Basil develops a strong bond with Tsuna, acting as a loyal ally, a dedicated training partner, and a good friend. Throughout the series, Basil shows considerable development as a fighter. Initially, while skilled, he is no match for a powerful assassin like Squalo. However, his own training and experience allow him to become a formidable combatant, skilled enough to serve as Tsuna's trainer. He is noted by Reborn to be several times stronger than a beginning Tsuna in Dying Will Mode. By the final arc, he is able to fight alongside Tsuna against powerful foes like the Vindice.

Basil possesses notable abilities centered around the Rain Flame, which has the property of tranquility. Unlike Tsuna, he can enter both Dying Will Mode and the advanced Hyper Dying Will Mode by ingesting special pills, a method that leaves his clothes intact. His primary weapon is a metal edge, a bladed boomerang that can be ignited with his Rain Flame. In the future, he acquires a Box Weapon, a Rain Dolphin he nicknames Alfin, which can use its tranquil flames to calm other box weapons and can synchronize with other boxes to launch special combination attacks.