Severus Snape glared at his empty classroom. She had just been in here, with her fifth year class. Her very presence had just fill this room, but he had to act nonchalant to her, even slightly cold, as if she was in other student. She hadn't been paying attention in last class, yet she had handed in her homework on time. As the class was reading, and she was whispering to a friend, he looked over her homework and could do nothing but grant her perfect marks. But she drew his attention and he could only watch her discreetly instead of grading any of the other homework the students handed in.
It was a lot easier to admire Ginny when he was teaching Potions, but now that he was teaching Defense Against the Dark Arts it was a lot harder. During Potions he would walk around and observe the all his students' while they made their potions, and only linger fleetingly at her cauldron to be able to smell her, hear her smooth voice. But now that he was teaching Defense Against the Dark Arts, it was mostly book reading and an occasional spell, there was no reason to walk around and observe, so instead he collected their homework and passed back the graded ones individually, lingering ever so slightly.
She reminded him of his lost love, and embodied every essence of Lily. Her red hair, her brain, her loyalty, her kindness, her scent, and even her love for Potter boys. And they were merely boys after all, unlike Snape himself. But no one knew of his secret love, his desire to just be near her. This was his second chance at love. At Lily Evans.
Snape realized, intellectually, that he could not truly have a second chance at love with Lily Evans through Ginny Weasley but he still let himself think of her as Lily. Sometimes he found himself referring to her, in his mind luckily and not out loud, as Lily instead of Ginny. He used to laugh during her first three years at Hogwarts when he even compared her to Lily when her looks had been the only thing that even close to mirrored Lily. But as she matured over the last two years she had become more and more like Lily herself, in her attitude and the way she carried herself.
In Ginny's fourth year was the first time Snape allowed himself to refer to her to himself as Lily. She had saw her laughing at the Gryffindor table with her friends, and for a moment he thought he saw Lily sitting here and laughing with her friends instead. It had shook him so much he had to excuse himself from the table. No one has ever said they noticed anything different about the way he treated Ginny compared to the other students.
He knew he shouldn't think of Ginny, his student, as Lily, but he still did. He allowed himself to fantasize, and live for his fantasy. He realized that he didn't even love Ginny, but instead he just saw her as his second chance at Lily. On a rare occasion he would imagine himself kissing Lily, then it would slowly fade in to Ginny. Sometimes, he even thought it was Ginny he fantasized about, but just used her embodiment of Lily as an excuse for his fantasies about Ginny. Sometimes he wondered who it was really he loved, Lily or Ginny.