Regulus of Hogwarts: Lord of the Stars

Lily spotted Regulus through the compartment window.

He started to give her a polite nod. She answered with a glare and kept walking.

For a second, he didn't get it.

Then he remembered.

Before the holidays, he'd told her he would write.

He had not written.

In his defense, quite a lot had happened. Training, Voldemort's gift, new magic, Germany, Freya, the ruins and bellatrix getting lit up. Somewhere in the middle of all that, a promise to a young girl had been completely shoved out of his head.

Across from him, Cuthbert and Alex exchanged a look and tried very hard not to grin.

The second Regulus looked over, both of them straightened up like model students.