Regulus
Regulus Black had been an animagus since he was 12. He did it in the summer between 1st and 2nd year of Hogwarts as part of the stupid Black family tradition. His brother had done it the year before and had transformed into a shaggy black dog. They laughed about the irony afterwards, but he was secretly happy with his brother’s animagus because it matched his: a small black cat.
The September after Sirius had transformed, he informed Regulus in a hastily written letter that his friends immediately wanted to become animagi too. Regulus didn’t blame them. There was a certain freeing feeling about being able to escape the world for a bit. He enjoyed becoming a quiet, unassuming cat, doing what he wanted without anyone realising it was him.
It took them ages to find the recipe, and to understand how to brew the potion itself, they were only 1st years after all. In fact, it took them until Regulus’ 2nd year and their 3rd to acquire the skills and ingredients to actually make the potion for themselves.
Regulus found a new hobby in watching them try and fail to convince the librarian to let them into the restricted section for months while snuggled up in his new favourite armchair which was hidden between two bookshelves and had a warm green blanket thrown over it. When they realised convincing the fed-up librarian wouldn’t work, they finally used their brains and resorted to magic.
He could have sworn he heard some hushed giggling one night in the library when he was curled up sleeping in his favourite armchair, because he couldn't get to sleep that night. But no matter where he looked, he never found the source. Perhaps Lupin was smarter than he had given him credit for and had managed to turn them all invisible with some spell.
Regulus still doesn’t know how they did it, but he got an excited anonymous letter one morning in the spring of his 1st year, in scribbled, barely legible writing. It read ‘we did it! I can’t believe we finally did it!!!!!! I’m not going to be home much for summer cause we’re gonna brew the potion then.’ He’s pretty sure it was from Sirius; the copious exclamation marks gave it away.
Sure enough, Sirius wasn’t home much that summer. When they went back to school, Sirus and his friends were calling each other ridiculous nicknames like ‘prongs’ and ‘wormtail’ which Regulus assumed must be related to their animagi somehow. Something else that changed, is that Remus didn’t seem to be getting as sick as he usually did every month or so.
Regulus made Sirius promise that he wouldn’t tell his friends about him being an animagus as well. But he was jealous of their silly nicknames. All he got was the occasional ‘Hi Reggie’ in the corridors or a quick ‘Morning Reg’ if they passed each other during quidditch practice. However, it did mean that he could go on his late-night walks around the castle undisturbed, and without getting caught.
It also meant that he could observe a certain someone in the library or playing quidditch stupidly early in the morning when no one else was awake. He even - rarely though, only when he was feeling particularly inclined to - accompanied him to the astronomy tower. Watching him gaze at the stars. But this was very infrequent, and Regulus was always confused why he even bothered. It was just James Potter after all.