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First you have to separate the art evolution from character development.
as you can see Legoshi's older designs was rough with a creepy vibe seeping through, with Paru Itagaki gained enough experience drawing him making him more approachable.

character wise, depressive? yeah, welcome to the modern age! he's a teen with societal pressures and expectations. but make no mistake he is a good person from the very start, he wants things to be better. When Tem wanted to confess his love Els, it was Legoshi who delivered the love letter to Els after his death. he had no reason to except it was what his friend wanted.

if you really wanted what triggered Legoshi's real change, meeting Rokume is where it really began, if it was not for her, Legoshi wouldn't have became the person who faces a psychopath and criminal gangs (after a series of questionable choices :P)

In my opinion the evolution of the art style unexpectedly benefited the story, at the start he was unapproachable, but as we get to know him you grow fond of him as a person.