Shion Sumeragi, affectionately nicknamed "Shii," hails from a family renowned for exceptional talent across diverse fields, including brothers who are a professional chef, pianist, and barista. As the sole daughter, she channels this background into a pronounced tendency to dote on younger girls, most visibly directed at club member Tamaki Kannazuki, whom she treats as a younger sister figure.

Standing tall at 172 cm, Shion possesses distinctive long violet hair and violet eyes, coupled with characteristically cool skin. She habitually wears her school coat draped loosely over her shoulders. Her outward calm and intelligence coexist with a significant deficit in common sense, frequently necessitating practical intervention from others, especially Kyōya Shinomiya, to curb her unconventional impulses—a consistent trait across all appearances.

Her genius-level intellect shines brightest in chess, where she secured multiple victories over world champions, a skill honed early in life. Despite such capabilities, she finds the label "genius" bothersome. Her interests extend to unconventional fascinations, notably a deep fondness for hamburgers that occasionally surfaces in dialogue. She voices pragmatic perspectives through statements like, "Being cold-hearted has its benefits too, see?"

Within the club setting, Shion maintains consistent interactions. She participates in daily group activities, often assuming a pseudo-sisterly role toward Tamaki, indulging in affectionate gestures like head-stroking. This dynamic underscores her desire for familial connection as the youngest and only daughter. Her core traits remain stable throughout the series and related media, showing no substantial evolution.