Much is unknown about Leorio's past. He once lost a close friend (a man named Pietro in the 1999 anime adaptation) to a debilitating illness. Leorio feels deeply remorseful about being unable to save his friend, as the illness was entirely curable but required a surgery he could not afford. Thus, he aspires to become a doctor — one whom not only cures the ill of their diseases, but also one who works pro bono for people who can not afford it. In this light, his outwardly superficial reason to join the Hunter Exams in the pursuit of great wealth gains a more noble justification — the money he earns is used not only to fund his medical school fees, but also to pay for the medicine and other miscellaneous expenditures when treating those who can not afford it.
Leorio is initially introduced as a selfish narcissist who places great emphasis on material wealth. Despite his assertions that he is only after money, he eventually reveals to Kurapika that his dream is to one day become a doctor in order to help the poor. He believes that all things in the world (lives, dreams, etc.) can be bought and that becoming a Hunter will provide him with the necessary funds for his medical education. Due to his first spat with Kurapika, as well as the test on the way to the navigator's cabin, it seems that he has a sensitivity for those who do not take him seriously or treat him with disrespect. His real priorities seem to be friends first, then his dream of becoming a doctor, with everything else afterwards.
Leorio is not one to balk and retreat when faced with overwhelming odds in battle. A loyal, trustworthy ally who greatly values the ties of friendship, he acts like the 'big brother' of the group. He often provides comic relief; Killua sometimes ridicules him for his comparatively older age, although he swears that his appearance is that of a mere teenager. His impulsive and brash nature has sometimes caused more trouble for people around him. Another flair he has is his ability to 'straight-talk' through many situations at hand.
At the tail end of the Yorknew arc, Melody claims that he has "the nicest heartbeat in the city," stating that "it's warm and gentle" and that he would make for a better doctor than a Hunter.