Kentaro Ichinose is the only child of Mr. and Mrs. Ichinose, who live in Room 1 of the aging boardinghouse Maison Ikkoku. When first introduced, Kentaro is a grade-school-aged boy, though by the conclusion of the story he has grown into a high school student. His family background is a source of quiet frustration for him, as he is often embarrassed by his overweight, loud, and alcohol-loving mother, and his father who is frequently absent due to work or unemployment. These disappointments lead him to feel that his parents are to blame for many of his struggles, including being teased by classmates for not taking long vacations or having a typical home life.

In personality, Kentaro begins as a typical bratty child who acts out against the boardinghouse's residents, particularly the main character, Yusaku Godai. However, as the series progresses, his reasoning and character are explored more deeply. He sheds much of his childish antagonism and evolves into a surprisingly jaded and observant young man. Far more mature than the chaotic adults surrounding him, he frequently functions as a voice of reason among the tenants of Maison Ikkoku, reacting to their antics with a cynical and weary demeanor that produces much of the story's humor. On one notable occasion, he becomes so disappointed in the immature behavior of the adults, including Kyoko, that he directly expresses his disapproval.

Kentaro's motivations are largely driven by a desire for normalcy and a quiet life, which stands in stark contrast to the constant parties and schemes of his mother and neighbors. He resents the poverty and instability of his family life, which prevents him from having the same experiences as his peers. Over time, he also develops a significant crush on Ikuko, the young niece of the apartment manager, Kyoko Otonashi. This relationship serves as a charming subplot that mirrors the awkward, early stages of the main romantic courtship between Godai and Kyoko, as Kentaro is too nervous to properly express his feelings to her.

In the overall story of Maison Ikkoku, Kentaro plays the role of a supporting character who often highlights the absurdity of the adult world. Initially, he targets Godai with his mischief, but as Godai matures and gains acceptance into college, the two develop a friendlier, almost brotherly rapport. Kentaro becomes one of Godai's key connections within the boardinghouse, and Godai shows him an understanding that his own parents often fail to provide, such as when he agrees to take him to the beach to make up for a missed family vacation. His key relationships include his distant but ultimately caring father, his boisterous and neglectful mother, and Kyoko, whom he respects and sees as one of the few sane people in the building. He also shares a notable dynamic with the mysterious tenant Yotsuya, once discovering an old photo album that suggests Yotsuya has not aged for decades, which deeply disturbs him.

Throughout the series, Kentaro undergoes a clear development from a resentful and acting-out child into a more composed, if still weary, adolescent. He transitions from being a problem for the residents to becoming a moral compass of sorts, even requiring his parents to prove they are sober by blowing into balloons. His presence diminishes in the latter half of the story, but he reappears in the final episodes to serve as a receptionist at Godai and Kyoko's wedding, now a composed high school student. Kentaro does not possess any special or fantastical abilities; his primary notable trait is his sharp, perceptive nature and his ability to see through the pretensions and foolishness of the adults around him, often with a single, well-timed sarcastic comment.