Kage Vincent - Vincent aka the Shadow King is Ichiro's older brother who caused the Light Kingdom's destruction. Ichiro's relationship with his brother is on negative terms as Vincent only sees Ichiro as a tool for creating havoc; Ichiro despises his brother for seeing him as nothing more than a tool and hates him for his abuse against him and his butler Sorrow.
When their parents pasted away, Vincent slowly began to act violently towards Ichiro and the people close to him. This was where Ichiro began to dislike his brother, telling himself that he never wanted anything to do with his plans. Ichiro's resentment of his brother only further increased when Vincent manipulated and erased Sorrow's memories. Ichiro snapped and opposed his brother by disappearing to earth.
Although Ichiro hates his brother for what he did throughout his childhood, Ichiro does not entirely despise his brother as he knows that something changed within him (hinting that Vincent's personality was different).
Yukimura Aiko - When Aiko was walking to school she witnesses Ichiro standing on top of a light post as he watches her but disappears before she is aware of his existence. He encounters her again after he saves her from a Kurayami, despite knowing the risk of being discovered. She offers Ichiro to live at her house while on earth and decides to trust him without any second thoughts. Thus leaving the impression that she was carefree and gullible. However, his view of her changes and they eventually become very close friends and are often seen together. She is very kind towards Ichiro dispute his initially cold attitude and comes to understand the hardships he's been through. They are both well known throughout the school which surprises Ichiro. Nevertheless he acknowledges and respects Aiko and values her as a friend. Both Ichiro and Aiko usually call each other by their first name.
Sorrow - [Ichiro's butler and first friend back in the Shadow Kingdom before Sorrow's memories were erased. However, their relationship now is quite strained. Sorrow proclaimed to be Ichiro's servant when they first met, which Ichiro strongly denied and insisted that they would be friends. After the death of his parents, Ichiro become insecure and think he was useless. Sorrow would always support Ichiro during these dark times, saying that he become who he wants to be.]
Mitsue Sayuri - A funny but very complicated friendship. After meeting Sayuri, Ichiro can sense the unfriendly feeling she gives off towards him. Their relationship started off quite bad, to be honest. It turns a bit worse because he hide the fact that he was living with Aiko, which was unintentional, however, over time, their relationship gets better.
Kawasaki Aoi - Ichiro meets Aoi after being forced to volunteer to help the student council. They appear to be in good terms with each other and share a friendly relationship.
Akanishi Ren - Ren constantly teases Ichiro and refers to him as "Prince-kun" while bowing to him as a joke. The two often bicker, but regardless, are good friends. They have a rivalry and compete with each other to see which one of them wins. When Ren reveals that she is Cure Blaze, Ichiro is surprised at first but puts the pieces together, saying that it makes sense.
Matsushima Taro - As the president of Aiko's fan club Taro is extremely suspicious of Ichiro's relationship with Aiko. Ichiro first meets Taro during his first day at Yusei and coincidently they are in the same class. Though Ichiro acts cold towards Taro because of his changing serious to fan-boy attitude, he soon opens up to him and they become friends.
Matsushima Tomiko - Tomiko is madly in love with Ichiro along with being his classmate and the president of his fan club. Most of the series' running gags revolve around Tomiko's over-the-top antics of pursuing Ichiro which often leads to misunderstandings. Still, Tomiko's affection for him is genuine as she is known for her unwavering loyalty and affection towards Ichiro. Ichiro considers Tomiko as a friend, thus ignoring her noisy statements and turning down her offers. She addresses him as "Ichiro-sama".
To be added.