Disclaimer: I don't own Harry Potter. This disclaimer applies to all chapters.

A/N: This is an AU, because a) I like AUs and b) the story I wanted to write wouldn't have been able to happen if it weren't. Also, this is doubly an AU because there is no Dark Lord and Petunia Evans will not marry Vernon Dursley.

A/N2: This is probably the only Lilybashing fic in existence. I haven't checked, but I've been reading fanfiction since early 2000 and I've never seen any.

A/N3: I will put new chapters up as soon as they're written, so don't nag. Also, this will not rise above PG-13 or T, as I cannot bring myself to write hetsmut.

The Slytherin's Guide to Dating

Step One: Make a Good Impression

She stood on Platform 10, waiting for her older sister, and fiddled with her dress irritably, wishing it hadn't shrunk in the wash. It had been a perfectly presentable knee-length dress, a bit low cut, but nice. Now it rode halfway up her thighs and clung to her slight curves as if it was painted on. She thought it made her look 'easy', but her mother had insisted that she wear something nice. She didn't know why, it was just her freak sister after all.

As she was ruminating about the unfairness of it all, she was watching one of the barriers carefully, waiting for the tell-tale shimmer that meant someone was coming through. A moment later it shimmered, but it wasn't her sister who came through. It was a tall, pale boy, just rolling up his robes and putting them in his satchel. She looked appreciatively; you didn't see a body like that everyday. His plain black t-shirt clung just enough to reveal a lean body, with muscles in all the right places, and his tight black jeans displayed an arse that made her nearly drool. His face wasn't too bad either, even if it couldn't hold a candle to his body; high cheekbones, stubborn chin, large nose and a pair of dark intense eyes... that were staring right at her.

He was having a truly bad day. He'd had a prank pulled on him that would actually have been amusing if he hadn't been the victim, he'd been outscored on his NEWTs but one by that arsehole Potter and, to top it all off, he'd run out of cigarettes. As he brooded about the injustice of it all, he glanced around, making sure nobody had seen him coming through the barrier. Oh damn, there was one Muggle, but she didn't seem the least bit astonished by the fact he'd come out of thin air, so he figured she must be waiting for someone. Taking a long look at her, he thought about going over and talking to her a bit. She was gorgeous, in a tight green dress that left nothing to the imagination and made her legs look miles long. He looked up at her face, and saw an odd smile on her coral tinted lips, and a pair of startlingly green eyes looking right at him.

She blushed, raised her chin determinedly and walked over. "Have you seen my sister? She should be here by now. Maybe you know her, she's always bragging about how popular she is; her name's Lily." Her voice dripped disgust.

He raised an eyebrow. "If you mean Lily Evans, she's making a prolonged and rather physical goodbye to her fiancé, James Potter." He scowled at the mention of his enemy.

"Well, isn't it just like the bitch not to tell us she was engaged, and to someone new, as well! My parents and I thought she was still seeing that hooligan Black." She was annoyed but resigned; it really was just like her. "Oh well, I'm Petunia Evans." She extended her hand, which he kissed, leaving her blushing.

"It is a pleasure to meet you, Miss Evans. I am Severus Snape."

"It's a pleasure to meet you, but please, call me Petunia." She was flustered, unused to the courtly and old-fashioned manners used by the handsome boy.

"Very well. Are your parents around, Petunia? We should warn them that your sister is likely to take quite some time." Severus smiled, pleased to be allowed the privilege of her given name so soon.

"I think they're in the cafe. They told me to wait for Lily while they ate." She scowled.

"Shall we go?" He offered her his arm, which she took tentatively.

Henry and Rose Evans were eating quietly when Henry looked and saw his youngest daughter approaching on the arm of a young man. "Rosie, look. Our Pet's got herself a lad."

Rose looked, and cursed herself for insisting that Petunia wear the dress. It really was too tight, but she hadn't wanted to be late to meet Lily after the end of her last year of school. To his credit though, the young man wasn't looking anywhere but her face, which had an unusually cheerful look on it. It was about time she got herself a boyfriend.

"Mum, Dad, this is Severus Snape. He goes to school with Lily."

"I'm Henry Evans. Pleased to meet you." Mr. Evans shook his hand heartily.

"I'm Rose Evans." She gave him a brief nod, then asked Petunia, "Where is Lily? I thought you were going to wait for her."

"It's my fault, ma'am." Severus interceded, rescuing Petunia from her mother's disapproving stare. "I thought that we should warn you that Lily's farewells to her fiancé have become rather extensive and that they still have not left their compartment." He deliberately implied that they were in a compromising situation, but they seemed more concerned that she was engaged.

Mrs. Evans gasped, "Fiancé?" and Mr. Evans said, calmly, "Well, Sirius finally proposed, did he? I thought he would, sooner or later."

"Sir, her fiancé is James Potter, not Sirius Black. She and Black broke it off in March."

Now Mr. Evans looked put out, and Mrs. Evans looked scandalized. "I would have expected her to tell me she'd gotten engaged to someone I'd never met." Mr. Evans said.

"She's engaged after only three months!" Mrs. Evans was horrified.

"No, madam. It has been five months since she began seeing Potter."

"But I thought you said she and Sirius broke up in March?" Mr. Evans looked puzzled.

"They did, but she and James had been seeing each other for two months prior." Severus was intensely disapproving.

Petunia, who had until then been listening quietly, spoke, "So Lily was cheating on Sirius? I'm not all that surprised. She always have to have it all. I haven't had a boyfriend she didn't steal. She hates my having anything she doesn't."

"Now, now, Petunia. She doesn't mean to steal them, she's just better looking than you are." Mrs. Evans was dismissive, heedless of the hurt written plainly on Petunia's face.

Severus was appalled. How could she condemn Petunia's looks when she was so obviously superior to her sister? Even near tears she was the prettiest girl he'd ever seen. He wrapped an arm around her slender waist and pulled her closer to his side. "In my opinion, Petunia far outshines her sister, but then, I've always disliked red hair. It's so incredibly gaudy looking. Now, if you will excuse us, we should go wait for Lily." He led Petunia away, with his arm still around her, leaving the auburn haired Mrs. Evans looking shocked and offended by the insult to her favorite daughter.

Mr. Evans looked tired, but was pleased that someone dared contradict his wife's opinion of Lily. She was a pretty girl, yes, but her personality left much to be desired and Rose was blind to that. "You went too far Rose. Saying something that hurtful to Petty was beyond the pale."

"What did I say that was hurtful?" She seemed honestly clueless and Henry gave up.

"Never you mind. Let's just go and meet Lily."