To me, a guy who risk using a potentially deadly move like the Twin-Maximum after seeing his friend cry definitely speaks volumes of how big his heart is. No matter what others may dislike about Shotaro (Which confounds to no end), his is the very heart of the show, representing the compassion that seems to lack from the other characters.
Philip lives and breathes on finding out what he doesn't know, and Terui does many things merely out of duty and (until quite recently) revenge. Shotaro, on the other hand, makes up for both of them by being the guy who helps people not for his sake, but for the people he helps.
Jimmy wouldn't have had the courage to go back to the stage if Shotaro didn't give him the moral support (no matter how bad Jimmy was), Terui would not have been as merciful as he was, Kirihiko would not have seen the truth if Shotaro did not share the same love he had for his city, and Phillip would have killed Akiko in FangJoker form if it were not for Shotaro's determination.
The list could go on, but the point is Shotaro doesn't need a reason to be kind, to be determined, to do selfless acts that would have end up hurting him, he just does because that is who he is. A big halfboiled hero, the detective that loves his city and the hate s to see its tears.