to analyze leorios character development from the hunter exam arc to the yorknew arc you first have to talk about the person he was/he was trying to be at the beginning.
obviously leorio is good from the core as confirmed by melody but at the beginning of and during the hunter exam arc he was at his lowest, putting up a facade and showing little care for himself.
his facade gets broken through quickly by kurapika, its one of a greedy, money loving idiot and how leorio sees himself and as such presents that way. leorio, as ive said before, believes his goal to become a doctor is a selfish one purely to relieve his own guilt. everytime someone compliments him he gets extremely embarrassed as though he believes hes not worthy of the praise.
so to come back to my original point, this first layer is how leorio sees himself, someone whos selfish and idiotic, greedy and weak. this facade is the one most of the fandom chooses to see sadly.
another prominent trait leorio shows in the hunter exam is his hotheadedness, particularly in moments where he feels as though hes being looked down on or when someone plays with the lives of others, most notably the old lady with her choice riddle and hisoka.
the encounter with hisoka brings me to my next point: his disregard for himself. especially during the hunter exam arc leorio shows just how little he cares about his own life as he faces down hisoka and lets hundreds of snakes bite him to warn kurapika and gon.
the snake scene is, in my opinion, crucial to leorios further development. kurapika and gon risk their lives to save and help him and that changes things for leorio. he realizes theres people who care about him, people whod miss him.
i still strongly believe in the leorio orphan theory and if it is true, he didnt have anyone before coming to the hunter exam, having lost pietro.
this care gon and kurapika show him is a turning point for him to become less reckless and he allows himself to show his caring side more. had melody read his heartbeat during the hunter exam, im sure it wouldve still sounded kind but not as calm and soothing.
thats how he was able to become the voice of reason in yorknew. its why leorio is the one most invested in keeping the group together. gon and killua are kids who dont think stuff like that through and ive discussed many times why kurapika isolates himself.
but of course this doesnt fix leorios problems. hes still plagued by guilt and a feeling of helplessness he tries to fight as its clearly seen in the 13th chairman election arc. he feels guilty and useless and does what he can to help gon.
hes selfless and kind and empathic and caring, thoughtful and wise but at the same time plagued by guilt, insecure and has little regard for himself.
his development from the exam to yorknew mirrors kurapikas development nicely and both of them feel guilty for letting the people they loved get taken away even though it was not either fault. they can help each other heal.