The Boy who got Tall use to be shorter than other children. He appears to have light skin, a green hoodie, a grey shirt, and dark jeans. Under the hoodie, he wears a vermilion cap. After being distorted by the mirror, he becomes extremely tall, being at a similar height to the adults of the universe.
Although he is childishly curious, he appears to be brave despite being cocky without minding the risks. Through his wish, he is seen kind and wanting to help other living beings.
He is later shown to care and work together with his friends, as seen by him taking down the Mirror Monster.
He is one of the children who entered a derelict house, with other two boys and the Humpback Girl. When the girl said to him a story of a monster in the house, he despised it, saying that haunted houses are just "kids stories".
Later, he wanted to become tall so that he could save helpless creature, play with unhappy children, and find lost things. When the Mirror Monster snatched the Boy who got Strong, he tried to save him but failed and later attempted to run away from the monster. After making a plan, he ultimately took down the Mirror Monster.