Inbetween Character: This character isn't as well-mannered as the nice character since he has a few negative traits. But unlike the mean character, he's doesn't have many bad manners as he does, and he's mainly a Jerk with a Heart of Gold with a heavy emphasis on "Heart Of Gold". Another variation would be that he isn't a jerk, he's just serious about his duties. He can be a Nice Guy, but not to the point of being naive or a pushover. He may be too introverted to be nice or mean, usually. Often, he will be the leader who shows respect to his comrades while keeping the two in line in one way or another, mostly the mean one.
Note that this group dynamic can be a unique way to establish moments of how each character acts and what defines them. This is a snippet in the Power Trio explaining the basic makeup of trios - "Trait 1, Trait-opposite-of-Trait-1, Trait-Between-the-Two."
From Gilligan's Island, Gilligan is the dumb, but nice one, The Professor is heavily prideful of his own intelligence, and The Skipper is a little nicer than The Professor; for he always loses his temper for Gilligan's incompetence, but does show he can be a nice guy every now and then.
We have the trio of Sonic, Tails, and Knuckles. Tails is the mild-mannered best friend of Sonic, Knuckles is the tough, hot-headed friendly rival, and Sonic mediates the two, combining the kind-hearted attitude with his cocky and snarky attitude (although it's mostly geared towards his foes).
In the OVA, however, while Tails is still the nice one, Sonic and Knuckles' roles have been switched, where it's Sonic who's a trademark Jerk with a Heart of Gold while Knuckles mediates where he's more friendly than his game portrayal but retains his seriousness.
In Mega Man X, X is the nice one who is pacifistic and a bit naive. Even so, Zero is the mean one who is snarky and cynical and rebukes X for his idealism. Axl mediates between, for he isn't naive as X, but doesn't have the same extent of having a shoot-first-questions-later mentality like Zero.
The Powerpuff Girls: Bubbles is the most nicest and gentle of the three girls, though she can come across as naive. Buttercup is the aggressive one who constantly argues with Blossom and picks on Bubbles, though she does have a soft spot on occasions. Blossom, the leader, mediates between the two, where she can be mild-mannered, but can come across as bossy at times.
In My Little Pony: Friendship Is Magic, we have the Cutie Mark Crusaders: Sweetie Belle is the nice one, Scootaloo the mean/tough one, and Apple Bloom the one who is the middle point between the two.
Ed, Edd n Eddy: Edd is the nice one, always trying to find a reasonable, non-violent solution and apologizing for his friend's behavior. Eddy is the mean one, always getting the other two involved in his scams and screaming at them when he doesn't get his way. Ed is inbetween, not as mean as Eddy, but more impulsive than Edd.
Thomas, Percy, and Toby from the earlier seasons. Toby is nice and respectful, Thomas is cheeky, arrogant and a bit troublesome, (although he has become nicer and kind-hearted later on the series). Percy mediates between the two where he can be nice, but has moments of being cheeky and troublesome like Thomas.
There's also the Terrible Trio of Gordon, James, and Henry. Henry is nicer and more sympathetic than the other two while James is more vain and arrogant than Gordon, who occasionally shows his soft and humble side more than James does.
There's the trio of Fry, Leela, and Bender from Futurama. Fry is the nicest guy of the group, albelt a bit dumb. Bender is the loud-mouthed, abrasive jerk who likes to drink and smoke a lot. Leela mediates between the two for she's calm and level-headed, although she would often appear as a nag to others if they did something immoral.
Theodore is the nicest of the chipmunks, serving as the Morality Pet to Alvin, who is an egotistical Attention Whore. Simon stays inbetween due to him being the smart one and his constant bickering with Alvin.
Their Distaff Counterparts, The Chipettes, are this, too. Jeanette is sweet-natured but shy, Brittany is hot-headed and selfish, and Eleanor is caring but more assertive than Jeanette.
The trio of Aang, Katara, and Sokka in season 1 of Avatar: The Last Airbender . Aang is easily the nicest character of the group, although he can be quite scary when pushed too far. Sokka is a full-on sarcastic, cynical, Insufferable Genius, but he does has his moments. Katara is nice and reasonable, but she has mood swings and can have a quite nasty, vengeful side when betrayed by those who were close to her.
There's also Azula (the mean one), Ty Lee (the nice one) and Mai (The inbetween one; mostly because of her Sugar and Ice Personality).
The Goodfeathers from Animaniacs. Squit is the friendly Na´ve Newcomer in contrast to Pesto, who has a violent temper and would always beat Squit up. Bobby is the leader and mediator often keeping Pesto in check or just get a good laugh at him beating up Squit.
The main siblings from Phineas and Ferb. Phineas is the most nicest and optimistic guy in the series aside from formerly being a Bratty Half-Pint from the pilot episode. Candace is always mean to her brothers and always want to bust them for their everyday activities. Ferb is inbetween, for he doesn't say much, although he'll speak a line at the end of every episode.
In Time Warp Trio, the main male trio has: Sam, the nice and nerdy one, Fred, the mean and tough one, and Joe, the inbetween and well-meaning leader of the group. For the female trio, we have: Freddi, the Shrinking Violet, Jodie, the snobby and needy one, and Samantha, the Brilliant, but Lazy one who remains inbetween most of the time. Jodie is the leader, despite being the mean one.
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it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
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