The hero is a peace maker. Smoothing over conflicts between warring people. And sometimes this hits closer to home.
Our hero has two sets of friends. They both like him, but they are unfamiliar with each other. Maybe they have never seen each other. Perhaps they have good reason to avoid the other. Ultimately the two meet, and it can get violent. Neither side is inherently evil, they just don't know each other. They may be trained to think of them as the enemy, or they might think they are a threat to themselves or to our Hero. Perhaps one side is a peaceful member of the side that the other demonizes.
Our Hero demonstrates his peacemaking skills and steps in quickly to smooth things over between the two. He introduces the two usually say something like "He's a friend." He shows that they are not enemies. This usually leads to the two sides making up quickly. Or not.
In short, two of the Hero's friends are fighting/aggressive, then the Hero steps in to introduce them to each other. Compare Character Witness
for when a less important character vouches for the hero.
- Young Justice's Collide has Robin assuring the rest of the team that an alternate dimension Jason Todd is a friend. Superboy was initially against the idea because Jason had earlier attacked Robin and "[he] don't like people who hurt [his] friends."
- Batman Beyond: Bruce's dog, Ace, takes some time to warm up to Terry.
- Jackie Chan Adventures' second season, when El Toro, a one shot ally from the previous season, turns up, he attacks Tohru (since the last time they met him was before his Heel-Face Turn) until the situation's explained.
- Jonny Quest TOS.
- "Werewolf of the Timberland". The wolf Gray One has just saved Dr. Quest and Race Bannon from being attacked by the "werewolf" (a man in a werewolf suit), but they don't realize this and Race Bannon prepares to shoot the wolf. Jonny and Hadji arrive and call out to them, telling them that the wolf is a friend.
- "Calcutta Adventure". Pasha Peddler shows up in his helicopter right after the team has been attacked by a mook in a plane. Race thinks he might be an enemy, but Hadji tells them he's a friend.
- Avatar: The Last Airbender: Sokka has to do this for Zuko twice during "The Boiling Rock," once for Suki, who remembered him from when he destroyed her village, and once for his father Hakoda.