Disclaimer: I do not own Harry Potter
There were two Remus Lupins. One was on display for the world, a quiet but brilliant kid who did his best to keep his friends out of trouble through loopholes. That Remus was smart and answered questions in class and almost never spoke out of turn. That Remus went by Lupin and liked to hide behind stacks of books and mountains homework.
The marauders knew a different Remus. They knew a Remus that would pretend to be studying but actually be munching on chocolate wearing an ugly old sweater while petting some random cat. They knew a boy who was witty and sassy and passionate and kind. They were friends with Remus, but respected Lupin as the authority figure he was. They recognized the distinction, but to them, he was just Remus, a relaxed boy who liked sweets and had a sharp wit.
There were also two subcategories of Remus Lupin. One was Moony: his wolf form. While Moony did like to bite people when given the opportunity, he would never hurt his woodland friends. Wormtail, Padfoot, and Prongs, were his friends and Moony would die to protect them. Moony was a combination of Lupin and Remus's personalities. He was the most powerful, and therefore in this form he was the alpha: Lupin's authority figure status. But he was also kind and fun to play around with: Remus's easy going nature. But one thing Moony was that Remus and Lupin were not was protective to the point of violence. He would not tolerate any threat to his small pack and he had ended up with a few scars because of it.
The last version of Remus Lupin was one only seen by Sirius Black. The guise of a lover. Sirius was very vocal about their relationship and Remus had no problems with that. But as much as he was amused by Sirius snogging him senseless in the hall for absolutely no reason other than "Your hair did that cute floppy thing." Remus would never drop his Remus persona. He was, with Sirius in public, Remus and not Lupin or Moony. But this private version or Remus Lupin didn't have a specific name. Sure Sirius liked to shout Remmy, or Love Muffin, or Buttercup and other such pet names but the side of Remus that Sirius saw when they were alone would never be called such stupid names. It was fierce like fire burning a passionate hole in Remus. He would never get rough or violent but he would become very tender, almost to the point where Sirius would become concerned. He would bear everything he had for Sirius in a display of raw emotions that was nearly overwhelming. Remus felt things very strongly whether it be joy or sadness or pain or love. He was a passionate creature in any form but his love for Sirius was its own side of Remus Lupin. A side reserved for none but Sirius. And when Sirius was with this private Remus he wouldn't joke or tease, he would simply smile and talk to Remus and kiss him softly. These kinds of moments were never seen by James or Peter or anyone else for that matter. They were purely for Sirius. This bare side of Remus was for Sirius. Remus would display his tangled mess of emotion and Sirius would pick a strand and unweave it for him. And that was how it went. Sirius would make Remus feel better, his emotions would be less painful, and he would breathe easier like a weight had been lifted from his chest.