What do you think Alexander's parents thought of Hephaistion? You've said before that you think Olympias disliked, or at least distrusted him, but what about Philip? Given that he didn't exile Hephaistion during the Pixodarus affair, that might implicate a level of trust or goodwill?
First, the fact Hephaistion wasn’t exiled owes to his age. Waldemar Heckel wrote an important article called “The ‘Boyhood Friends’ of Alexander the Great.” In it he points out these weren’t boys. I wrote them as boys in Dancing mostly to reduce the number of names floating around (and give some familiar ones, like Ptolemy and Nearchos). But I point out in my “Author’s Note” that they weren’t, in fact, Alexander’s age at all.
These were actually adult advisors, and Philip blamed them for the bad advice they gave Alexander, in encouraging him to intervene with Pixodaros. The youngest of them were probably Ptolemy and Harpalos. Nearchos, Erigyios, and Laomedon were men, probably around the same age as Kleitos. Ptolemy and Krateros were contemporaries, for instance. Again, I de-aged them in the novel for my own purposes.
NONE of Alexander’s younger syntrophoi were exiled, so it’s insignificant that Hephaistion wasn’t.
The infamous letter from Hephaistion to Olympias recorded in Diodoros may not be real. It’s precisely the sort of thing that was invented later. That said, it may (probably does) reflect tension between the two, probably for ultimate influence over Alexander. Again, in my novel I push it back into their youth, but it’s impossible to say when it started. As I hope I was able to explain (particularly in that little extra scene, “Two Scorpions”) but also in the first chapters of Rise, Olympias’s concern is Alexander’s public standing (timē). She’d care a whole lot less if he’d been the younger. Later in the series, I intend to expand that to her resentment that Alexander listens to Hephaistion’s advice more.
Finally, and this is purely part of the novel’s characterization of them, part of why they clash is that they’re a lot alike. That’s also why Alexandros is drawn to Hephaistion, even though he doesn’t quite recognize it. I’m not sure Hephaistion really does, either, but Kleopatra does/will, as does Kampaspe. But they’re on the outside looking in. Makes it easier to see patterns.