The Nice Guy (or Girl) is nice. Not that he'll necessarily be an All-Loving Hero, but he's definitely not unpleasant, obnoxious or an overly cynical Byronic Hero edgy rebel. These characters are kind, friendly, psychologically well-balanced, morally good and socially decent in most cases, and in short, someone anyone would genuinely like to be around and have as a friend, which is often what they are to the hero and/or his extended cast. He won't engage in jerk-like behavior, but probably responds in kind to Vitriolic Best Buds, and Beware the Nice Ones is still absolutely to be considered.