Harry Potter and Neville Longbottom, both of Gryffindor, both born on last days of July. Their destinies were thought worlds apart, but it turns out that Neville could very well have been The Boy Who Lived.
Neville is someone Harry feels very sympathetic about: his parents, his terrible time in Potions, losing Trevor at various points, the remembrall incident, et cetera, while Harry is someone Neville looks up to and someone that teaches him to stand on his own (Dumbledore's Army, "you're worth twelve of him" about Malfoy, etc.).
Both characters are in every book (and every movie). Harry is, of course, the main character of the series, the hero. Neville is rather a minor-almost-major character (and I would say major now, after OotP) who has truly shown his mettle time and time again (e.g. trying to stop the trio from venturing out at night in SS, helping Harry against the Death Eaters, etc.). They've both experienced loss, in one way or another (they both can see the thestrals; Harry's parents are dead, Neville's are permanently in St. Mungo's, etc.). They're both amazing in many ways, and I love both their relationship and the potential of their relationship (all aspects). And I'm looking forward to the rest of the series and seeing whether Neville becomes closer with the trio or not, begins standing alone more, etc.