He is always on the lookout for any potential danger around Hermione. In many fanfictions, when they are exploring an old wizarding castle or something similar, Fred is constantly scanning the area, ready to jump in front of Hermione if any harm comes their way. He also stands up for her in social situations. If someone questions Hermione's intelligence or her Muggle - born status, Fred is quick to defend her with sharp words and a fierce look.
Fred might also offer Hermione magical protection items. In some fanfictions, he gives her a charmed amulet that warns her of approaching danger. And he'll physically shield her from spells or attacks. For instance, if there's a rogue wizard casting spells at them, Fred will put himself between Hermione and the danger without a second thought.
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.