In some fanfictions, Naruto shows his protectiveness by getting extremely angry when someone badmouths Sakura. He won't tolerate any disrespect towards her and will quickly defend her honor. He might also use his influence in the village to make sure Sakura is always safe and well - cared for.
Often, Naruto's protectiveness is shown through his actions. He could be seen accompanying Sakura everywhere, especially in dangerous missions or when she has to go to uncharted places. He'd offer his strength, whether it's his ninja skills or his knowledge, to keep Sakura out of harm's way. He might also share his food or supplies with her first if they are in a scarce situation, just to ensure she is okay.
Minato might show his protectiveness by always being near Naruto in spirit. For example, he could leave behind some kind of jutsu or power that activates when Naruto is in great danger, like a hidden seal that releases a powerful defense.
She might physically stand in harm's way. For example, if there's a threat approaching Naruto, Kushina would put herself between Naruto and the danger. Just like how a mother bear protects her cub.
In many such fanfics, Percy uses his intelligence. For example, he might study the laws and regulations of the wizarding world thoroughly to find loopholes that can protect Harry from unjust punishments or accusations.
Zoro often puts himself between Luffy and danger. For example, when facing strong opponents, he'll be the first to step forward and engage, making sure Luffy is safe behind him.
Fred often uses his humor to diffuse tense situations that Hermione might be in. For example, if she's being harassed by some bullies, he'll make a joke that makes the bullies look stupid and also makes Hermione feel better.
Sasuke might show his protectiveness by always being near Naruto in dangerous situations. For example, if they are in a battle, she would quickly jump in front of Naruto to shield him from attacks.
Often, in such fanfiction, Itachi protects Naruto from the Akatsuki. Since Naruto is the jinchuriki of the Nine - Tails, the Akatsuki might target him. Itachi, being part of the Akatsuki but having his own motives, would ensure Naruto's safety from his comrades. He could mislead them about Naruto's whereabouts or lie about Naruto's power level to keep them from making a hasty move against him. Itachi might also protect Naruto from the villagers' negative attitudes. Naruto was often ostracized in the village, and Itachi could use his influence or just his presence to make the villagers think twice before treating Naruto badly.
She might be confused at first. Sakura is used to Naruto being friendly and now his coldness would be a big change. She may start to question what she did wrong.
In some fanfictions, when Peter is facing a powerful villain like Thanos, the Avengers immediately surround him to keep him safe. Iron Man may fly in front of Peter to take the brunt of an energy blast. Another instance could be when Peter is being bullied in school. Hawkeye might secretly keep an eye on him and step in if things get too rough, using his skills to intimidate the bullies without revealing his identity.
Naruto often shows his care through his actions. He might use his healing techniques on Sakura if she's physically hurt. For instance, if she has a cut or a broken bone, Naruto could use his medical ninjutsu skills, which he has learned over time.