Peter is aching because he feels he's being replaced by Sirius, and Regulus is aching because he feels like he's being replaced by James.

They both build walls around themselves out of fear of getting hurt but go about it in different ways. Regulus is cold, shutting everyone out and hurting them before they can hurt him. Peter comes off as this happy and open person when, in all reality, he has a perfectly curated personality for each person in his life.

I like to think they first start spending time alone after Peter finds Regulus alone in the greenhouse, gripping a letter he wrote to Sirius but won't ever send. And they just.. sit there in comfortable silence. Neither one pressing the other but offering their presence as a I'm here for you.

I am about to list an ungodly amount of headcannons.

It was Regulus who fell first, but Peter fell harder.

They're both transmasc but only out to their friends for a variety of reasons.

Peter smoked cigarettes, but Regulus hated the smell, so he stopped. They both smoke weed in the greenhouse with Reg's friends (Barty, Evan, Dorcas, and Pandora).

Peter got along with all of Reg's friends but was particularly close with Evan because of shared nerdy interests.

They have a shared notebook that they write to each other in. They're both great at art and love drawing each other in the notebook.

They don't fight a lot, but when they do, it's always explosive. Shouting, crying, name calling, etc. It eventually gets better, but their first big fight caused them to not speak for days.

Peter is the first to apologise, Regulus spends longer apologising.

Almost never do PDA. Are always touching in some way, shape, or form when alone. Pinkies hooked together, thighs touching, sitting on each other, etc.

When Peter joined the deatheaters, that was the first time Regulus started secretly questioning the Dark Lord. Something about seeing the love of his life fall down the same path as him started making the pieces click together in his head.

After Regulus' death, Peter mourned for the rest of his life but couldn't even show it because his friends never knew about their relationship.