At some point between the end of sixth year and the beginning of seventh, she wasn't quite sure when, Lily had become James Potter's girlfriend.
At some point, during sixth year, she had become friends with James Potter. It started when he had found her late one night upset in the common room. They had spent the night talking, and she had been able to release all of her worries on him. She realised then, that the war that had been going on since she was in first year, was effecting him too. No, he didn't know exactly what she was feeling, she being a muggleborn and being targeted far more than him with his pure blood and rich connections, but he was there and he was worried but he was strong and he was willing to fight. He had learnt about the grey inhabiting the space between the white and black. He became someone she could trust with good judgement someone she could rely on to help her and to get her through, someone with a shoulder to cry on when it all became too much. Someone to fill up the void that was looking her best friend and confidant.
At some point, James Potter stopped being there to fill up the void. He began to carve his way into her heart, leaving a gaping hole with his name graffitied all over it. It was a big space that he built himself in her heart, one that was large enough to fit in his big head and even bigger heart, but it was his own space, and he was there to stay.
At some point, James Potter had invited himself into her house. He had turned up during the holidays and met her mother and sister. He visits became more and more frequent and she would live for the days that she would see him. He would take her out, on adventures to parks, to woods, even to places as exotic as her back garden. He seemed to be able to make an adventure out of everything. Sometimes, he would bring the marauders, and they would take her under their wing. They would help her sneak out and they would all get really pissed, and it would be great and perfect and lovely and absolutely amazing to be able to finally relax, to be able to get away, just for a few hours, from the deaths that were written in black in on the back pages of the prophet.
At some point, one of them kissed the other. They kept it a secret, something that they hid from her parents and his friends. It was something pure and holy and a ray of light to them in the darkness that they were surrounded by. It was something that they didn't want tainted by hate. It was difficult at first, but they learnt to act like friends in front of people, while they were lovers behind their backs.
At some point school started. James became the head boy to Lily's head girl, and they found that they were not able to spend as much time together as they wished to. Their relationship was kept a secret, and it meant that they savoured every moment they had together. Patrols were taken advantage of, and they would often be the last left in the common room at night.
At some point, they were caught. On the settee in the common room, early one Saturday morning, they awoke to the face of one Sirius Black. It was no longer a secret then, there didn't seem a point, and by the next morning it seemed that everyone in the school knew. They still didn't hold hands in public, and they certainly did not spare a small peck. They had decided that there were things that they wanted to keep, to themselves, as the relationship that they had tried to protect from the harsh words, was now being subjected to them. If a stranger were to see them, they would have believed that they were merely friends, but those closest to them knew that their relationship was so much more.
At some point, a pregnant Lily Evans became Lily Potter, in a ceremony that, just like their relationship, was only known to their closest friends and what little family they had left.