I'm a Ravenclaw, through and through, but ever since this occurred to me, I've not been able to totally shake it: if you want to understand the true power of friendship through the lens of Harry Potter, look to Slytherin.
For those less well acquainted with Harry Potter, Slytherin is one of the results for the magically assisted quazi-Jungian personality profiles routinely done to children in magical Britain. They represent temperaments driven by different clusters of virtues, for the most part. More information can be learned here. Our main characters are in Gryffindor – exemplifying bravery, chivalry, and other largely marshal values that become more relevant as the background war plot becomes more important over the series.
Despite one of these archetypes being hard-working team players, I don't think that truly captures the power of friendship. For instance, modern mega-organizations exemplify the virtues of lots of people working together, as well as hard work – but to say a modern mega-corp captures the power of friendship would be laughable.
The best example we have of Slytherin's ambition and cunning is perhaps Horace Slughorn (as other prominent Slytherins are somewhat distracted by their strange secondary interactions in the war that grows to dominate the series). He collects people, building a social network to put anyone else's to shame. And not just to drop names – he collects and distributes favors and introductions when he can, and is famous for his after-hours social club. Any promising wizard or witch becomes a fascination of his, a way to extend his social network along with his influence and power. But what is that power used for? Mostly getting sports tickets and having the ear of newspaper editors. Nothing startling, of course – any real imposition would endanger his ability to call in more favors later.
He doesn't want to dominate the world, he just wants his fingers in lots of pies, and he knows how to earn his place in people's lives. He's useful to them, if only because access to his social network is valuable, and it isn't a value he hoards. He's willing to give people a path to their dreams (like when Harry Potter had to abandon his dream of being in law enforcement until Slughorn changed requirements holding Harry back), because what's a more surefire way to have the most useful of friends?
All of that being said, it's important to remember how much pointless strife there was in those children's books. Ron stopped talking to Harry over the Triwizard Tournament misunderstanding, an utterly petty grudge driven by Ron's jealousy and distrust. But for Slytherins, their jealousy and distrust are much more natural. What would a young Slughorn have done? Oh, be the most supportive friend he could to the young enigmatic champion, of course. Having a friend lose wouldn't be nearly as useful as having a friend who won.
In the first book, Hermione, upset after overhearing Ron casually insult her pretentious help in class, locks herself in a bathroom (where she is later attacked by a Troll). Dare I say, that would be pretty unusual in Slytherin – at least one cunning classmate would see someone without many friends, but whip-smart, willing to help others, and see a fantastic investment of time and friendship.
Time and again I see Slytherin as capturing the true power of friendship. It's all academic for me, being a Ravenclaw (double meaning very much intended), but a story exploring this would do wonders for children (and adults) with that particular form of wit.