Enter the sidekick.
Ron is basically the Chewbacca to Harry's Han Solo: his confidant, his best bud and his fellow traveler on the road to adventure. On the surface, there's not much to Ron beyond his unkempt red hair and pale skin. But the closer we look at him, the more we understand why he makes such a great BFF for the Boy Who Lived.
Born Into It
In the first place, he was born and raised in the Wizarding World. He knows all about their hidden lairs, he views magic use the same way you'd look at washing the dishes, and everything Harry sees with such wonder just feels like Tuesday to Ron.
Dumbledore and McGonagall could help…but they're adults and therefore not to be entirely trusted. Harry needs a buddy to show him the ropes. That's a job for Ron, who's got that whole magical thing covered, and can give Harry the skinny without making him feel like a teacher's pet.
Ron's also approachable. He has a bit of a scruffy underdog vibe, just like Harry does. He's a wizard, sure, but his family comes from the wrong side of the tracks in his world: workaday magic-users who don't get invited to all the cool parties like the Malfoys do. He uses a hand-me-down wand, his brothers already have the whole place scoped, and his "magical" animal companion is Scabbers the rat. Ew.
Apparently being a wizard doesn't solve all your problems.
Ron may have 99 Scabbers-related problems, but having a negligent family ain't one. In later movies, the Weasleys practically adopt Harry as one of their own, but even in these early days, their kindness shows us just how important Ron will be to Harry's life.
Look at the way Mrs. Weasley gets Harry through Platform 9 ¾:
MRS. WEASLEY: Not to worry, dear. It's Ron's first time to Hogwarts as well. Now, all you've got to do is walk straight at the wall between platforms 9 and 10. Best do it at a bit of a run if you're nervous.
It's pretty clear that Ron's mum is kind and sympathetic, which gives us the knowledge that Ron is also going to be a big-hearted bro.
Ron's struggles at Hogwarts mirror the struggles that Harry faces. This includes the snazzy stuff—like getting past Fluffy or handling the Wizard's Chess game—but it also includes issues that everyone can relate to, like studying for hard classes and steering clear of terrifying teachers like Snape.
The fact that they're brothers-in-arms bonds the two boys together in ways that no mentor or teacher ever could. They come from different worlds, meet on common ground, and eventually start learning just who and what they are together as equals.
You can tell how much that means to Ron, who's had a family to love him but has been lacking in the buddy department. Initially awed by Harry's rep, he soon figures out that Mr. Potter is just as pants-peeing scared as Ron is, and they group together like best buds always do.
And as Harry's friend, he shows the qualities that Harry has so desperately needed in someone—anyone—all his life. Ron's loyal, sympathetic and willing to set his fears aside for the sake of his chum.
Seriously, look at what he does in that final game of Wizard Chess:
RON: Do you want to stop Snape or not? Harry, it's you that has to go on. I know it. Not me, not Hermione, you.
When he sacrifices himself in order to allow Harry to win the chess match and face down Voldemort, Ron's being more than just a bro. He's being tested. And like Hermione, he passes with flying colors: not just in his raw skills, but in his willingness to take one for the team.
Whatever sunset Harry's headed for, this grinning redhead is going to be with him every step of the way. And Harry, more than anyone else, knows how much of a difference a good friend makes.