As a boy, Simon is physically underdeveloped for his age (early teens), standing at little more than 5-feet tall. He wears a long sleeved blue and red jacket and brown shorts, topped with a pair of yellow and red goggles and brown work shoes. He has indigo hair and wide eyes with small pupils, and his iris color was eventually revealed to be an almost the same color as his hair. Simon also ties on shin guards and sports a cloth waistband, and the Core Drill hangs from a fastened leather string around his neck. Originally, Simon wore a dusty brown cloak, which eased his passage through the narrow tunnels of the underground, but this cloak was blown off by the wind during the initial escape from Giha Village.

Though valued as one of the most skilled diggers in Giha Village, Simon is ignored by the adults and snubbed by the girls, who consider him disgusting because of his peculiar habits and odd smell. Lacking parental figures to help him mature, along with his fear of earthquakes, Simon is plagued by low self-esteem and has many cowardly tendencies. Even after his friendship with Kamina, he continues to act cowardly and doubts his own abilities. However, Kamina's inspirational speeches gives Simon just enough confidence to endure his fears and to accomplish something. Young Simon is also susceptible to bouts of intense determination, such as when he charged at the Gunzar (later, the Gurren) and the Enki.

In Part II, Simon is thrown into a maddening, pervasive depression, driving him to be overly aggressive in combat toward his enemies. Although the entire Team Dai-Gurren is mired in low morale, Simon is particularly affected as he feels he is personally responsible for the tragedy that befell the entire team. His psychotic outbursts and pessimistic comments further isolate him among the Team, even as some members reach out to him. Simon's meeting with the mysterious Nia helps him come to a revelation about his own nature, leading to him putting aside all fears and doubts in favour of courage and confidence. He also manages to come up with his own original inspirational speeches, though some team members feel he needs some work.