Lennon is the son of the archaeologist Jidan and the witch Atelia, making him the half-brother of the protagonist Arusu. His parents' union was forbidden under the laws of the Witch Realm, and he was separated from his father while still a child. Growing up without a stable home, he became a pirate roaming the Interdimensional Sea, where he ambushed vessels that were transporting failed witches to the Human Realm, taking their food and supplies. This solitary and harsh life shaped his personality: he developed a deep distrust and dislike for magic and the political structures of both the Witch Realm and the Warlock Realm. He is blunt and does not hesitate to speak his mind when criticizing witches or expressing his beliefs. At the same time, he can be cryptic about his past, revealing details only when it serves his purposes.

Lennon's primary motivation is to survive and to oppose the systems that caused his fractured upbringing. His role in the story begins when he uses the True Book of Spells to summon Arusu to the Magical Realm, acting as a catalyst for her adventure. He later finds Arusu aboard a ship meant to deport failed witches and calls himself her mirror, implying that he reflects her own journey of being an outsider. He provides practical guidance and support, though often from the margins, and helps her navigate the dangers of the realm.

Key relationships include his mother Atelia, who was forced to leave him and Jidan to return the True Book of Spells, and his father Jidan, who spent years searching for him. When Jidan is imprisoned by the warlocks, Lennon remains unaware of his father's fate for much of the series. His bond with Arusu is central: as her half-brother and a fellow child of a forbidden union, he shares a connection that becomes important as the true nature of his family is revealed. At one point, warlocks capture him, mistaking him for Arusu, which forces a confrontation between the witches and warlocks and compels Atelia to confess her past.

Throughout the series, Lennon undergoes development from a cynical, self-reliant pirate to someone who reconnects with his family and becomes part of the larger struggle to protect the Magical Realm. His abilities are not magical; instead, he relies on the skills of a pirate, including combat, navigation, and ambush tactics, as well as his sharp observations and ability to read people. He is physically characterized by short blonde hair and green eyes, and he often wears gloves and carries a facial mark. His presence in the story underscores themes of exile, family secrets, and the consequences of prejudice.