He is somewhat tall for his age, since he is two years younger than Sabo but is still about the same height as him. He wears a shirt with frills going down the middle along with a bow-tie and suspenders. He is often seen smiling. He has a round head with a thin, flat nose and a bowl-shaped haircut.
Like most Nobles shown from Goa kingdom, he is an arrogant person who does not care for anyone that is of lesser status than himself, referring to them as trash. This extends to referring to Sabo as "stupid" and "trash" due to what he heard from their parents, all casually, disregarding any manners or respect. However, Stelly can put up a facade of good manners and devotion in front of his foster parents. He is also not very brave, crying in fear when Sabo attacked him, and shunning any responsibilities when Sabo ran away again. In short, he is the complete opposite of his older adoptive brother. Stelly also seems to be intelligent, according to his teacher and adoptive parents, due to his higher status upbringing, and Sabo's father desiring him to be the back-up heir.
Stelly was adopted into being a noble by Outlook III since he had shown lots of promise. When Sabo is brought home by his father, Stelly introduces himself to him. Stelly later visits Sabo in his room telling him if he will not succeed then he would replace him, becoming the new successor. He also tells Sabo that he had the devil's luck, if he had stayed in the Gray Terminal, he would have died. Sabo grabs Stelly's shirt and demands him to tell him everything. Stelly tells Sabo that only if there is a fire at the Gray Terminal, then the kingdom will be clean of trash.
Stelly seems to resemble Helmeppo in many ways. For example, they are both cowards, they both have the same hair cut, they are both extremely arrogant and have large egos. However, this only applies to an earlier Helmeppo, as he later became more brave, changed his hairstyle, and became more modest and generous.