Kaito Daimon stands as the narrative's central figure. His distinctive appearance features messy brown hair partially tied in a short ponytail and layered dark brown eyes. He typically wears sunglasses perched in his hair, a red unzipped vest over a sleeveless black shirt, baggy green pants with straps extending to the knees, and red shoes with black socks. This attire later evolves to include a sleeved shirt.

Personality-wise, Kaito exhibits a generally uncaring attitude toward rules, frequently engaging in prohibited activities like constructing mazes from school desks or ignoring warnings. He communicates with blunt directness. His primary passion lies in puzzle-solving, where he demonstrates exceptional concentration, intuition, and insight, especially with complex challenges. He uniquely perceives the emotional intentions of puzzle creators, viewing puzzles as entities deserving liberation through solving. This fuels his intense aversion to life-threatening puzzles, stemming from his parents' death during an attempt to solve one.

Sent to Crossfield Academy in England following his parents' demise under Baron Kaido's guardianship, Kaito formed a deep friendship with Rook Banjou Crossfield. They bonded over puzzles in the forest, with Rook creating them and Kaito solving them, mentored by Jin Makata. A pivotal childhood incident—failing to meet Rook on time due to his tardiness—resulted in their separation and instilled in Kaito an obsessive commitment to keeping promises. This profoundly drives his actions to prevent similar betrayals and protect loved ones.

A critical development occurs when Kaito discovers and contracts the Orpheus Armband during his first encounter with a philosopher’s puzzle. This artifact augments his brainpower under extreme stress, enabling a heightened cognitive state called "Phi-Brain" where he visualizes solutions. However, this power causes physical consequences like fainting and exhaustion. The armband evolves across seasons, later showing him apocalyptic visions of potential futures if he pursues certain paths. These visions lead him to reject the armband’s influence twice, shattering it by Season 1's end. He retains his accelerated puzzle-solving abilities without it.

Key relationships define his journey. Nonoha Itou, his childhood best friend, provides unwavering support, managing his daily needs. Kaito demonstrates protective loyalty, shielding her from danger while bluntly criticizing her cooking, labeling her sweets as "weapons of mass destruction," often earning physical reprimands. While her romantic feelings are hinted, Kaito’s reciprocal emotions remain ambiguous, though he consistently prioritizes her safety. Rook transitions from cherished friend to antagonist under the Orpheus Armband’s corruption, becoming obsessively hostile. Their relationship undergoes redemption arcs, with Rook ultimately sacrificing himself for Kaito and later returning as a conflicted ally burdened by guilt. Freecell, initially concealed as "Minotaur," harbors resentment, blaming Kaito for his mother’s heartbreak and death. After confrontations, Freecell shifts from vengeance to seeking reconciliation, becoming a trusted comrade. Gammon Sakanoue begins as a competitive rival, driven by jealousy and a desire for monetary gain. His insecurities culminate in betrayal when he joins P.O.G. to oppose Kaito, though he later seeks forgiveness and reintegrates.

Kaito’s skills center on his prodigious puzzle-solving prowess, recognized by peers and adversaries. His title "Einstein" reflects this expertise. Notable quotes include "Puzzles don’t endanger lives," "There’s no puzzle I can’t solve," and "It’s puzzle time!" Trivia notes include the pun in his name—"Daimon Kaito" approximating "mondai kaitou" (problem/solution) in Japanese—and his dislikes of green onions and Nonoha’s sweets, though he gradually tolerates the latter.