Dick Grayson, easily. In the Golden Age, Robin’s arrival slowly* helped Batman to give up murdering every criminal in sight. (I guess Bruce finally put his pistol away when Robin said, “I want a gun! Chicks dig the gun!”) And even though later continuities removed Batman’s year-one bloodlust, they stuck with the idea that Dick helped Bruce to mellow out. Perhaps the best take on this development was in The New Frontier, when Darwyn Cooke had Superman notice a major change in Batman’s costume and demeanor upon taking in the little orphan acrobat.
*I say “slowly” because in Detective Comics #38, Batman tells Dick, “My parents too were killed by a criminal. That’s why I’ve devoted my life to exterminate them.” And then the two of them spend the rest of the issue killing gangsters to avenge the deaths of the Graysons. It would take a few issues before the Dynamic Duo stopped leaving corpses in their wake. And people say that Frank Miller’s ASBAR got it wrong! Pfft!