Nanami Sai. The mathematician and half-brother of Nanami Ryusui, son of the Nanami conglomerate family. He constantly corrects people, insisting he is a programmer. A natural mathematician and programming genius, his talents were so exceptional that they caught the eye of his father and the Nanami clan, who tried to force him into becoming a dedicated fixer for the family. He considered fleeing to India to escape Ryusui and the conglomerate to become a university teacher, but ultimately gave up on that plan, choosing instead to walk slowly by Guest's side at a pace befitting his age. Suffocated by the oppressive control of his father and the Nanami family who wanted to exploit his talent, he discovered a love for gaming and began secretly teaching himself computer programming on the side. Raised under the shadow of an incredibly influential family name, he is very timid and introverted, but his passion shines when he is around things he loves. He finds standing in front of people quite burdensome and is particularly uncomfortable when Ryusui praises him as a 'mathematical genius.' He tends to look away from things that overwhelm him. When Sai was young, while he was shrinking back after being struck on the hand, he was led out to the garden by Guest to play in the rain before anyone could stop them. This became a precious memory and the reason he holds Guest in his heart. Occasionally, if he is prevented from programming, he exhibits the eccentric behavior of grabbing pens in both hands and scribbling down pages of machine code. This suggests he is ambidextrous. Furthermore, Ryusui's assessment of him as a mathematical genius seems accurate, as he can solve complex arithmetic that would take others hundreds of times longer in just a few minutes or even seconds. He has brown eyes and hair, and is of mixed Japanese and Indian descent. He is considered handsome and is quite tall. He prefers wearing white dress shirts with the sleeves rolled up. He has good eyesight.

Nanami Ryusui. The legitimate son of the Nanami conglomerate, who describes himself as a man of enormous greed. He laughs heartily, saying 'I want it! Haha!' about everything, and openly displays healthy desires, such as investing in others. He speaks with phrases like 'Is it not so! Haha!', 'This is getting interesting!', and 'Indeed!' To this man, 'desire equals justice.' As soon as he saw his half-brother Sai's extraordinary talent for mathematics as a child, he followed him around constantly, pestering him to build racing tracks or play chess, simply wanting to play with his brother. Despite being the legitimate son, he was even excluded from family photos on the grounds that he didn't fit the Nanami conglomerate's image. Since Sai faced similar treatment, Ryusui feels a strange sense of kinship with him. He is so obsessed with sailing ships that he asks Sai every day, every moment, every second to calculate mast formulas and travel distances based on wind direction, or what would happen if a miniature model were scaled up to 1:1 size... Unlike their father, Ryusui truly wants to see his brother's talent flourish out of pure intentions. However, this is so relentless that Nanami Sai now recoils at the mere mention of Ryusui's name and constantly tries to escape and hide from his younger brother. He has blonde bobbed hair, brown eyes, a hearty smile, and thick eyebrows.