Disclaimer: Still not mine.
Fifth Year
Their fifth year began with promise. He and Lily had finally convinced Slughorn to let them have a copy of the Sixth Year Potions book. They had told him that they wanted to get a head start on his N.E.W.T.S. class, and he had relented because he knew that they could pass their O.W.L.S. blindfolded.
And so they began. Together they modified the Potions textbook (honestly, who wouldn't think that crushing the sopophorous bean would produce more juice?), and in its margins, Severus was putting his talent for Defense to the test. He had been displeased, not only with the quality of the professors who changed from year to year, but also with the quality of spells that they were taught.
He argued back and forth with Lily on the subject. She believed that what they were being taught was appropriate to their age and skill level. He argued back that if that was true then why were they correcting the Sixth Year Potions book? A satisfactory conclusion had yet to be reached, but Lily eventually consented that the curses and spells that Severus was inventing were genius and very useful.
But the year took a downturn when the fights between Severus and the 'Marauders' increased in frequency and viciousness.
Lily didn't believe him when he told her that Black had set up that prank in order to kill him, and that Potter had been in on it, but had backed out at the last minute due to cold feet. He could tell even then that she was slipping further away from him. She was disgusted by his other 'friends' in Slytherin house. But what she didn't understand was that the changing political climate in Slytherin made it impossible to sleep safely at night unless you were 'friends' with the right people.
No, she was a sheltered Gryffindor who hadn't a single clue about the things that Severus had done to keep her safe from his fellow Slytherins. Instead she was happy to worship Potter, the boy who'd 'saved' his life. And if things weren't already bad enough, Lily had to go and let slip to one of her friends a spell of Severus' own creation from the book of the Half-Blood Prince.
And on the same day, people in the halls were being strung up by some invisible force, and since the spell was cast nonverbally nobody could tell who the culprit was. Unless those prats, the Marauders, were in the hallway, and then everyone knew who to blame.
"How could you Lily? Those spells were for us! Not for those foul little gits!"
"It was an accident Sev! She caught me practicing it one day on a rat and she wouldn't shut up until I told her! I didn't know that she would spread it all over the common room!" But he knew that she was just making excuses. That particular friend of hers was known as one of the biggest rumormongers in Gryffindor house. But how could he not forgive her? After all, she was still the only friend he had.