The Nice Guy (or Girl) is nice. Maybe not a saint, but definitely not unpleasant, obnoxious or an edgy rebel. They're friendly, psychologically well-balanced, morally average (if not better) and in short, someone anyone would 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.
Want to know what writers think of that? "Boring!" This is because they can be conflict null zones romantically and dramatically because of a perceived (or real) lack of depth. Along with the Happily Married couple, the Nice Guy is among fiction's "favorite" Friendly Targets. They'll die senselessly to motivate their hero buddy, lose the girl to the more dynamic but unstable Dogged Nice Guy or just in general go through constant torment from the world around them.
On the other hand, if done right, it can demonstrate why a guy has beautiful women attached to him: he really is a remarkably wonderful and compassionate person. The Nice Guy can be engaging, likable or The Protagonist; all it takes is giving them a personality outside of "nice" such as: a scholarly refinement, a chivalric attitude, some engaging relationships, and a pinch of Hidden Depths. Having the right actor with the thespian skill to take those character qualities and wield them with charm and sincerity can help, which is how master actors like Christopher Reeve and Chris Evans took archetypal Superhero paragons of pure goodness who superficially seemed bland and colorless, Superman and Captain America respectively, and made them compelling characters audiences loved.
Nor does it mean being nice means they're not tough, or an Iron Woobie. If combat intellect or combat ability are involved, it's akin to Obfuscating Stupidity or Crouching Moron, Hidden Badass because you naturally expect the dark, tormented loner to be rough and tough, and the nice guy to be a Non-Action Guy. So when it comes time for "No more Mr. Nice Guy", heads roll. On the other hand, with the All of the Other Reindeer trope in play, the nice fellow can be a redeeming factor as a person whom the beleaguered hero realizes is on his side and lets him know he's not alone. Unless the nice guy is the beleaguered hero, then he's screwed.
Often, authors go the abbreviated route and give them a Dark and Troubled Past triggered by a Berserk Button. Either way, you can expect their niceness to be in spite of whatever tragedy or drama is in their lives; kind of an equal and opposite reaction to it. If done too suddenly, it implies a level of secrecy and acting that a Stepford Smiler would be jealous of. When done well, a more troubled character will rebuff their advice or friendship with "What do you know? You've never had X happen to you!" to his "I did", thus revealing healed-over scars.
Furthermore, the Nice Guy trope can overlap with Innocently Insensitive, in which case the character can be just as insufferable at times as the Jerk with a Heart of Gold or even the Jerkass, but not at all mean-spirited like them. However, if the setting is comedic with many Chew Toys around, you can't always count on them to be the sympathetic ones.
Another trait Nice Guys often exhibit is being Easily Impressed. Most people like to be complimented or appreciated so writers will often give the Nice Guy low standards to give a mediocre or upset character a boost in morale. Unfortunately, doing this will often make them incredibly reckless or inflate their ego a bit too much.
Occasionally, however, the treatment goes the other way around: Perhaps in a world set upon Black and Grey Morality, and Gray and Grey Morality, this Nice Guy actually stands out because he IS a Nice Guy, he doesn't have the excuse to be cynical or mean, nor does he need to have a tragic backstory. He's that nice, friendly and helpful... that he eventually becomes a fan-favorite due to how much he stands out amongst the more cynical cast members, a refreshing breeze to say.
If he does get the girl, it's Single Woman Seeks Good Man. Usually, the result of Upbringing Makes the Hero. Though watch out if you've pressed his Berserk Button. Exact oppositions of this trope are: Jerkass, Lack of Empathy, Jerk with a Heart of Jerk and the Complete Monster (extreme opposition). If the villain is a Nice Guy in spite of his evil deeds, then he qualifies as Affably Evil.
Fortunately, this one trope has many Truth in Television examples. So many, in fact, that there's no need in listing them here, thanks in advance.
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