He didn't notice Stiles' crest-fallen expression when the younger boy came into the deserted classroom and saw Derek leaning against Ms. Blake's desk, a small but sincere smile stretching unused muscles in the Alpha's face. Derek hadn't really seen a lot of Stiles lately, what with the Druid and Alpha Pack taking up their respective attentions, though he'd heard through Scott about how Stiles had figured out the deaths were very specific. It was strange how the boy had gone from being an irritating all-consuming presence to barely a blip on his radar in a matter of months. He pushed aside the crazy notion he missed him.

"So you think you'll come out with me tonight?"

"I can't, Derek. It's a school night!"

Her scandalized whisper was really a flirtatious ploy, her face stern even as long slim fingers toyed with a strand of her hair, winding it round and round. Her lashed dipped to hide her dark eyes, a slight blush gracing her high cheekbones. He marveled at the slight dusting of freckles spread across her nose, and wondered if the small imperfections would taste as sweet as her lips. He hadn't had the chance to compare the two, but he dreamed of it more than once in the past few weeks. The last dream had featured just red lips and a star-burst pattern of moles that demanded his attention.

"Uh, Derek...uh hi. What-what're you doing here?"

The unexpected voice startled Derek into nearly slipping from his position on the desk, and it was only then he realized how close he'd gotten to Jennifer as he'd subconsciously closed the distance between their faces. Frustration clawed at him for a moment when Jennifer jerked away at the intrustion and hustled to the board to finish wiping it down. The tense line of her back showed how uncomfortable she was having a student discover them in a somewhat compromising position. Of course, it was just Stiles who wouldn't say a word, if only because he was afraid of Derek. Okay, maybe afraid wasn't the right term as their mutual life-saving had dulled the antipathy they'd started with; it was hard to fear someone who fought to keep you alive. And even if Stiles were afraid, he wouldn't let it stop him from doing what he believed. It was one of the things Derek liked...er tolerated about Stiles the most.

"Mr. Stilinski, I didn't realize we had an appointment."

Derek whipped around to show red eyes to the boy, and found Stiles wasn't looking at him but at his own feet. He looked gangly and awkward in a way Derek didn't remember him being. If anything, the boy had come into his own over the summer as if he'd started growing into the promise of his long rangy body. Even his hair was longer and fuller, framing his narrow face in an attractive way. Or at least Derek assumed as much since he'd smelled Lydia Martin on him nearly every time they'd come into contact with each other; it was as if Jackson leaving had opened her eyes to the boy who'd always loved her from afar. Even Derek, who tried to stay out of the teens' lives as much as possible, had known of Stiles' decade long crush and had mentally scoffed at such devotion for a shallow girl. Sure she was smart, but did she really deserve Stiles' heart?

"Yes, Stiles. What're you doing here?"

Despite Jennifer knowing about his "monthly" problem, he hadn't really clued her about the breadth and depth of the supernatural world in Beacon Hills. He wasn't used to liking a pretty girl, and his previous experiences in this area had resulted in almost his entire family burning to death in a fire. Yet, there was something about Jennifer that was disarming. It weirded him out a little at how comfortable he felt with her, as if he'd known her a long time instead of a span of weeks.

"Uh, I saw you through the, uh door, and uh, stopped to say hi, but uh, didn't see you were, uh, you know...busy."

The word sounded wrong dripping from Stiles' mouth, as if he'd witnessed them in flagrante delicto.

"You two know each other?"

Jennifer's surprise was real and not entirely unexpected. Derek hadn't revealed the true identies of the two wolves he'd fought in front of her, and had purposely let her think they were feral nomads - which wasn't too far from the truth - who'd attacked him. Nor had he explained his status or the problems he was facing. Even when he'd stumbled to her car after his fight with the Alpha Pack, letting her draw him into the warmth of her home and heal beneath her kind hands, he didn't disabuse her of her belief it was the same wolves from before who nearly killed him. She'd turned into an avenging angel, hellbent on saving him from the world, and he'd been charmed enough to kiss her in gratitude. She was the first person to have his back without question in a very long time.

Well, maybe not the first. Stiles, despite his often blood-thirsty nature, had put aside his loyalties to Scott in order to help him out. It wasn't the same though.

"I'm his Big Brother."

It was surprising how easily the lie slipped out. Stiles stopped twisting his hands in his overlong sleeves and looked up, his mobile mouth quirked into a sideways smirk and Derek relaxed at the familiar expression. Stiles had an impish sense of humor and often loved playing pranks and jokes on people, even if they never realized it. Then Derek berated himself for thinking Stiles would give up the game; if he wouldn't tell his father, there was no way he'd let a near stranger in on the secret.

"Yes, I'm his Little Brother. We've been together for a while now."

The unusual emphasis he placed on together made Derek feel strange, but it must've just been him because Jennifer had turned back, her eyes soft with affection. She looked pretty in her red dress and loose curls, pale skin displayed in a way that drew his eyes. He coveted the opportunity to lay her on his bed and strip her, learn her slender body with his tongue and teeth until she writhed with pleasure.

Stiles slipped up beside Derek and nestled into his side, startling the older man from his increasingly warm fantasty. It was odd at how natural it felt for him to be there, but Derek supposed his wolf had accepted him as pack because there wasn't a hitch or a grumble from his inner beast.

"You must feel like such a lucky boy to have a strong Big Brother."

Stiles' dark eyes traced over Derek's face then further down as if cataloguing him, pale fingers brushing against his biceps as he simpered, "I wanna be Derek when I grow up. He's so manly."

Derek's playful growl wasn't as playful as his expression suggested, and by Stiles' stiffening he knew it too, but Jennifer fell for the act hook line and sinker.

"Well, I'm sure Derek is a wonderful role-model."

Jennifer couldn't see Stiles rolling his eyes and Derek wanted to bite at him, push him against he wall, but knew he couldn't show his dominance in front of her. Even flashing red eyes while she looked was off-limits and Derek just felt frustrated at his inability to stop Stiles from whatever this was.

"I'm sure you have to leave now, Stiles. The Sheriff is surely looking for you."

It was a pointed remark to the younger boy. Scram, kid. Adult time here.

"You forgot about promising to help me with my Advanced Chemistry homework. You know how hard it is for me," Stiles said, taking a step away with down-turned lips. It wasn't quite a frown or a pout, but some weird amalgamation of the two, making him look quite pitiful. It wasn't something that worked on Derek, but he forgot he wasn't the only recipient of the Look.

"Stiles -" he snapped, but was interrupted by Jennifer's heartfelt "I don't mind if you need to go Derek. We can take a raincheck on our date."

Her face turned as bright red as her dress when she realized she'd said something so personal in front of a student, but Stiles had on his angelic expression now that he'd gotten his way.

"Don't worry, Ms. Blake. I didn't hear anything," he promised with a roguish wink. "Meet you at the car, Derek?"

Jennifer's presence was the only thing stopping him from making good on the threat he silently promised in retribution. Stiles grinned, his whole face lighting up with glee, and Derek's breath caught for a brief moment. It wasn't often he saw true happiness on the younger boy's face. It had taken him awhile to realized Stiles was a superb actor who used words and his mobile face as props to shield his innermost thoughts. It irritated Derek though he didn't understand why. Why should he care about some scrawny sixteen-almost-seventeen year old's thoughts and feelings?

"Call me later?"

The tone of her voice shook Derek from his reverie and he realized he was still staring at the empty doorway Stiles had passed through.

"Yes."

"Does he, uh, you know, know?"

It was endearing how awkward Jennifer could be at times. One moment she was poised and feminine, then the next she stumbled over her feet and sqwuaked like a startled chicken.

Derek wasn't sure how to answer. Part of him wanted to be truthful and lay out everything for her perusal; it was the right thing to do when starting a new relationship. Then the cold pragmatic part of him reminded him the last time he felt like this and bile rose in his throat. Jennifer was nothing like Kate at her most perfidious, but old habits died hard. Plus, it wasn't really any of her business how Stiles figured into his life. At least not now.

"No, Stiles doesn't know anything."

She nodded, a conspiratorial smile turning up the corners of her mouth as she processed the implication of his words. Derek experienced a moment of déjà vu, as if he'd seen that expression before, but he really hadn't. They were in the first stages of courting and hadn't spent too much time together, so he couldn't have memorized her expressions yet, but she looked familiar somehow.

"Talk to you later," she promised breathily, and Derek turned to go, mind already racing as he thought up different ways to torture Stiles slowly for his interruption.

He had a long and enjoyable evening planned ahead.

A/N: Title is from the literal translation of Doppelgänger. In case there is some confusion, Big Brothers-Big Sisters is a national volunteer youth mentoring organization. So when Derek proclaims himself Stiles' Big Brother, he doesn't mean it literally.