It was striking how often in HBP we were told that Lily Evans Potter was highly gifted at Potions. As far as I recall, this talent was never mentioned in previous books. We know that her wand was "good for Charms," according to Ollivander (who should know). We know that she had red hair, was Muggleborn, was Head Girl and a Gryffindor, and that she had green eyes like Harry. She was one of the few people who had "defied" Lord Voldemort three times. She was married to James Potter. We know that she was capable of great, self-sacrificing love for her child, and we know that for some mysterious reason, Voldemort gave her an opportunity to live. We know that she didn't like to see people bullied ("Snape's Worst Memory," OotP). And, thanks to post HBP interviews with Jo, we know that she was popular, and that James wasn't the only one who fancied her.
The fact that she was good at potions isn't, perhaps, so significant in itself. If Dumbledore chose her as Head Girl, it stands to reason that she was a good student. It's the fact that she seems to have been amazingly good at potions that intrigues me. Slughorn remembers her very particularly and almost rapturously. As Harry begins to shine in Slughorn's class (with the help of the Prince), Slughorn compares him to Lily over and over. Not to Snape, mind you, who is the Half-Blood Prince and has been Hogwarts' Potions Master for years, but to Lily. In fact, at one point Slughorn even says, "not even Severus..." Now THAT really intrigues me! Snape's notated book shows an amazing talent for potions, as well as a fairly alarming aptitude for creating and perfecting curses. How on earth would one think of adding a clockwise stir, or crushing instead of slicing except through an incredible instinct for how ingredients will blend? It reminds me of a great chef who intuits the particular interactions that might take place by the slightest change of treatment of the ingredients at hand. But Lily was better than Snape at Potions?
While I was first reading HBP, I thought that perhaps Lily was the Prince. I couldn't work out how, of course, being that she was Muggleborn and not a male. However, the constant references to her potions skills must mean something. Lily and Snape were in the same class, and yet Lily is the one that Slughorn remembers as the Potions maven.
The only interaction we have ever seen between Snape and Lily is the pensieve scene in OotP. That was in their fifth year, as they had just finished their OWLs. The book with Snape's notes all over it is a NEWT level book, so it had to be used in their sixth and/or seventh year. In the pensieve Lily "blinks" and stops for a moment when Snape calls her a Mudblood. Her shock is understandable. She stands up for someone who turns around and insults her horribly. Granted, Snape is in the unenviable position of being bullied and humiliated, which would make anyone cranky (perhaps especially to the person who tries to "save" you). Still, "Mudblood"???
Incidentally, and this is a tangential thought, but I doubt it will fit into any other editorials that I may write, so I am including it here: I hope that Harry soon realizes that what he saw his father doing to Snape was, after all, merely payback for what Snape evidently did to others. Snape takes full credit for the spells that Harry learns from the Potions book. He says that he invented them. Levicorpus, which James uses on him, was apparently something that Snape had created and used on others. Lupin says a lot of people used that spell and that it was en vogue at the time. But Snape is very proprietary about it, and the other spells in the Potions book. Also, the spell that Snape attempts to use on James, which cuts James' cheek before Sirius' warning - I wonder if this might have been Sectumsempra? It makes a fierce cut on James' face, although the spell didn't make full and proper contact. It's possible that what James was doing was giving Snape a taste of his own medicine. It changes my idea of James' choices in that scene, and makes me wonder if he was, while certainly not blameless, perhaps less of a nasty bully than we might think in that moment. If Snape created those curses and used them on other students, he may have been "justly served," as it was "a poison tempered by himself." (Laertes, Hamlet, Act V)
Back to "Snape's Worst Memory." Lily may also be surprised because she doesn't expect that behavior from Severus for some reason. She immediately turns on him and calls him "Snivellus" after he calls her a Mudblood. (Lily is clearly not a pushover!) Perhaps she is surprised because they haven't had the same animosity towards each other that Snape and the Marauders share. I have read the theories that explore the possibility that Snape loved Lily. It's certainly a possibility, though most of the evidence to back it up is negative evidence. Lily doesn't seem to expect an insult from him. He doesn't insult Lily to Harry throughout the series the way he insults James. If he indeed felt remorse after sharing the first part of the Prophesy with Voldemort, the reason may be that it caused Lily's death. He shows no regret or sadness over Sirius' death, so I find it hard to believe he cared about James' either.
While I don't find enough canon evidence to draw a conclusion that Snape loved Lily, it is certainly possible. I think it's also possible that, since they were both exceptional at potions, and since Lily seems to have been a fair, open minded and kind person, they may have worked alongside each other in NEWT Potions and developed some sort of accord. Perhaps they had a bit of competition going, or perhaps they even helped each other. Perhaps a mischievous Lily was intentionally kind to Snape, just to annoy James.
There is one tiny moment in HBP that makes me wonder whether Lily may have made some of the contributions to Snape's enhanced Potions book: Hermione says that she thinks the writing looks more like a girl's. I don't believe there is evidence to show that there is more than one person's writing in the book, so this is probably a red herring. But it is interesting. What if Lily's talent is what is in Snape's old book? Even if she didn't voluntarily make the entries, Snape might have observed and copied her. Whatever the means or reason, the fact is that Snape and Lily must have been in the same NEWT level Potions class (assuming Lily took NEWT Potions - and that does seem to be probable, based on Slughorn's evidence). We know how good Snape is at Potions, and Slughorn remembers Lily as being the brightest of the bright - and better than Severus.
I cannot imagine that this was thrown at us only for the purpose of misleading us as to the identity of the HBP. I think that Lily's Potions talents will be significant in Book Seven. I don't know how, but I think they will. Potions played a huge part in HBP. The cover for the adult version of the book was, in hindsight, one of the biggest clues we got about the contents of HBP. We saw the only Potions Master we have ever known switch to teach DADA, and a new and mysterious Potions Master in his place. The entire Half-Blood Prince part of the plotline was based on Potions and the Potions Book. We also saw Malfoy's use of the Polyjuice Potions to disguise Crabbe and Goyle; the Felix Felicis Potion and all of its contributions to the outcome; and, of course, the vile green potion that poor Dumbledore drank for the greater good of the Wizarding World. I would HATE to think that Lily had ANYTHING to do with that potion.
I wonder if it was something to do with Potions that made Voldemort give Lily the opportunity to save herself? Maybe he hoped to use her? Maybe Snape wanted to save her and suggested to Voldemort that he might be able to use her? Could Snape have even been at Godric's Hollow with Voldemort that night? Could he have intended to spirit Lily away after James and Harry were dead? Is there some reason why Voldemort would have given Snape a "favor" by sparing Lily? (Hard to fathom Voldemort doing anyone a favor, but there might be a reason.) Whatever the outcome, I think that we will hear more about Lily and her remarkable instincts about Potion making. I only hope that what we learn will somehow help Harry with the daunting tasks that he has yet to face.