Honestly? I've read a lot of these, and the subtle, unsentimental stuff always hits harder. It's never a dramatic 'I'll die for you' speech. It's Blaise Zabini, who everyone writes as this aloof rich kid, casually mentioning to a group of Ravenclaws that Potter's flying technique during the last match was textbook perfect, shutting down some sly dig about his seeking being all luck. It's Pansy Parkinson, of all people, overhearing a younger Slytherin making a crude remark about Harry's dead parents and freezing him out of their social circle for a week with a single, icy glare—no words exchanged, but the message is crystal clear. The protection is woven into social politics, a currency they understand.
Then you have the Daphne Greengrass archetype, the so-called 'Ice Queen'. Her version of protection is strategic information sharing. She'll slide a note across the library table with a single line about which corridor Filch is staking out tonight, or which potions ingredient Slughorn is suddenly low on, because she knows Harry and his friends are about to brew something questionable. It's presented as a transaction, a favor to be called in later, but the timing is always a little too convenient to be purely selfish.
And of course, Snape. Fanfiction that explores a more nuanced, reluctantly protective Snape is a whole genre. It manifests as brutal, targeted critiques in Potions class that are actually (badly) disguised lessons, as docking points from other Slytherins who get too boastful about the Dark Lord's return, and as brewing a specific, complex antidote that just happens to be in the hospital wing the night Harry gets hit with a nasty hex. He'd rather swallow Veritaserum than admit it, but the actions pile up. That kind of writing makes you squint and read between the lines, which I find way more rewarding than overt declarations.