Disclaimer: I do not own Inuyasha or anything you might recognize in this story.

To readers: I apologize for the very long, multi-year delay in getting this out. Truly not a good reason for it in sight. But I can say that I feel that chapter 4 was too rushed and their relationship developing too fast for my tastes and I just didn't know how to continue it. The story's tone has changed a bit in this last chapter but I hope you'll find it enjoyable.

This is the last installment of Reaction Mechanism. Thank you for reading!

Reaction Mechanism Ch 5

A tinkling sound came from the other side of the room, rousing the dark haired girl from her slumber. Rin's eyes fluttered open and she groaned, rubbing the sand from her eyes. Blearily, she groped for her alarm clock. It was 11am. She sighed, rolling onto her back and wrapping her blankets tighter around her body. Her phone made the same tinkling noise again. With a displeased grunt, she pushed the blankets off herself and swung her lean legs out of bed. Half stomping towards her desk, she swiped her finger over the innocent phone to turn off the reminder. She had planned to go shopping with some friends but backed out last night when she realized it was going to snow. Problem was, she forgot to turn off the damned phone's reminder.

The dainty young woman turned her head towards her beckoning bed, but shook her head and forced herself towards the bathroom. Winter break had just finished four days ago and she was back to Eastern Standard Time, so she may as well try to get used to it. She made a disgruntled sound as she mechanically moved her toothbrush – break had been way too short and now she was back to the grindstone known as medical school and once again 2600 something miles from home.

It was well past noon by the time Rin had polished off her oatmeal and got dressed to go out for a walk. She happily breathed in the crisp cold air of the snow covered outdoors and allowed her eyes to dance over the whitened rooftops and driveways. She loved walking in fresh snow – something about it was very refreshing. Something in the back of her mind niggled at her to go back in the house, grab her dissection atlas, and head to anatomy lab to review the brain her group had been working on the day before yesterday but she resolutely stomped the idea down. New Year's resolution be damned! She wasn't going to spoil a perfectly good day by getting herself drenched in the smell of formaldehyde.

Instead, the now 23 year old young woman leisurely strolled through her new neighborhood – well, new as of one and half years ago. Not many people were out, probably because of the snow, but here and there she could see little children rolling snowballs that were only slightly smaller than themselves and couples sitting nearby watching over their kids. Rin blew a stubborn strand of hair away from her face, which was already reddening from the chill. Her warm brown eyes were downcast. This sort of tranquility was seldom seen in the bustling Morningside Heights neighborhood where she lived during undergrad.

Fleetingly, the image of a painfully handsome man with striking silver hair swam past her. She felt a twinge in her chest. She wondered, certainly not for the first time, how Sesshou- no, how Professor Tsukiaoi was doing. Yes, Professor. She firmly thought to herself. They weren't on a first name basis. Not anymore. Not after what she had done. But she still thought of him as Sesshoumaru – and she thought of him frequently. The number and quality of his research publications had not diminished in the slightest since she left his laboratory – if anything, they had increased. Career wise, the man had just been awarded his second Lasker Award and she wondered if he was aiming to kick someone off the 10 youngest Nobel Laureates roster. She was more worried about how he was faring on the inside.

She chuckled to herself while shaking her head. Of course he'd be fine. She sometimes thought too highly of herself and of her position in the man's heart. After all, he didn't really care for her. He cared for some other Japanese woman who also happened to be named Rin – who miraculously bore a stunning resemblance to her. No, that wasn't right either. More accurately, it was her who was a dead ringer for the Rin that Sesshoumaru loved.

She had always been suspicious of how much Sesshoumaru knew about her. The spring semester of her senior year had kicked off with a flurry of fancy dinners, Broadway shows, operas at the Met, and random walks in Riverside Park on the arm of her gold-eyed ex-professor turned boyfriend. Hardly a day went by without the two of them going on some sort of date or just talking to each other about the miniscule happenings of that day. Which was surprising, since she never pegged Sesshoumaru for the talkative or caring type. Rin had spent the good part of the semester simply glowing while her lips were quirked in a perpetual smile and her heart would go into worrisome fibrillations when she was in Sesshoumaru's warm, safe, and strong presence. At first she was pleasantly surprised each time he would bring her a favorite food during exam cram sessions or guess what she was thinking about. She thought of it as destiny.

Then the questions started. Her roommates, in not so brusque words, asked why he had chosen her out of the multitudes of beautiful and successful women he could have had. At first she brushed it off in her naivety. She had, of course, heard in length about relationships, but it was her first time dating someone and the reality that the rose-tinted glasses were just that – glasses, not truth – didn't sink in. Then she looked around. Why had he chosen her? She was a young, hopeful undergrad without a career or anything to offer him other than her feelings – and even in that department, it seemed that he outranked her. And while she was sure there were gold diggers out there among the throngs of women out to get him, she was equally sure there were those who valued his intellect, his thoughts, his personality, and… well… him.

She had struggled with herself. She was being silly. After all, love didn't need a reason. Did it? She felt as if she were Bluebeard's bride – tempted to see that forbidden room. Funnily enough, Sesshoumaru did have a forbidden room in his overpriced penthouse overlooking Central Park. It was the room opposite Sesshoumaru's bedroom, and when asked what it was for, the man had so skillfully changed the subject that she didn't realize he had until after she got back to her dorm. For any normal circumstance, she would have understood that the man needed his private space. But for Sesshoumaru, who left his home office open for her to freely thief and sell off his research projects and steal his credit cards and cash – not that she ever did—it was surprising. So one night after they'd had a sumptuous dinner, made, surprisingly, by the man himself, she finally couldn't fight off the self-questioning and low confidence that had been building up and opened Pandora's Box.

Inside were lifetimes of paintings, pictures and memoralia. A charming young woman in a traditional Japanese uchikake sat beside a man, who was undoubtedly Sesshoumaru – dressed in the formal dark haori and hakama. The same woman laughing while Sesshoumaru rested his head in her lap. Another wedding picture – this time in western style in what was no doubt a grand Italian cathedral. The two lovers – the married couple – in a gondola in midday. The images went on and on. Some painted by a masterful hand reminiscent of some Da Vinci, others in an impressionist style impressively similar to a nameless Monet. Still more taken by what seemed to be an old black and white box camera. But in each, the handsome man she'd come to love was unquestionably happy and in love. And the woman looked as if she were Rin's twin. Though, Rin could not fathom herself with that look of pure bliss, mutual understanding, and love on her own face.

She still remembered the feeling of sharp pain tugging at her heart, as if threatening to pull it out of her chest. Her throat felt as if it would close off and she would suffocate to death. She vaguely heard the silver-haired man's beautiful voice call out after her in alarm and distress. She might have even felt his warm strong arms clasping her small frame within them before she slapped and punch anywhere she could with her hands still covered in tears and – she thought rather mutinously – snot. At least she ruined a good jacket.

Rin brushed some unbidden tears from her cheeks, now ruddy from the chilly air. She missed him. She had chosen a medical school he hadn't written her a recommendation for. It wasn't the best one she'd been admitted to, but at least here nobody knew she was connected to him. But she thought of him – and often. Maybe if she had given him time to explain… She cut off that thought. Explain what? That she was a worthwhile substitute? But from time to time, like now, Rin felt an inexplicable urge to just be by his side even if he took her to be someone she wasn't.

More tears. She raised her hand to angrily swipe them away. Before her slender hands, already cold from the wet tears and winter chill could reach her face, she felt a comforting warm finger catch her tears as if they were pearls to be treasured.

She gasped and looked up, though she really didn't need to, because she already knew what she would see. Or rather, who she would see.

But like good food, a favorite location, fun activity, or any other unforgettably beloved entity, memories did not do reality justice. Her memory could not fully capture the silky sheen of his silver crown, the golden specks in his amber eyes, the lushness of his lips, or the warmth of his solid, toned body against her own as they hugged.

It took a long time for her to realize what was happening – like placing a man who had gone without water in a desert for several days in front of a stream – she starved for this presence too much to notice the lion on the other side of the stream. But when she came to her senses, she still held still and lived in the moment.

"I found you," her gold-eyed man whispered.

His voice tugged at her heartstrings. But she pushed against his firm chest.

He tightened his embrace, "Please," he murmured.

"I'm not who you take me to be," Rin choked on her words, afraid he would let go now that they were voiced.

He did loosen his grip, but just enough to look her straight in the eyes.

"I was wrong," he enunciated steadily, though his hands were shaking, "you are you."

"Then why are you here?"

He looked puzzled and surprised. Then, a forlorn look dawned on his immaculate features.

"Rin – I…I never…"

Rin raised an eyebrow. "Never took me for her?"

His shoulders slumped. "I did," before Rin could stop him, he continued quickly, "but I never wanted you to become her."

Rin didn't know what to say. Through the last three years, she'd fantasized about him chasing after her – then promptly scolded herself for it. In her fantasies she enacted plots in which he would swear his undying love for her – not that other woman, or say that it was all in the past. She never thought he would find her and admit that he did substitute Rin for the other Rin.

"Can we start over?"

Rin wrinkled her brow – she couldn't quite process what he meant.

"I am just a widower who met you," he started, she unconsciously leaned her head towards his chest, listening to the soothing rumbling reverberations of his chest as he spoke, "and neither of us know anything of the other," he was murmuring now and she closed her eyes to live the story he was telling, "you are a completely new person to me – someone unlike my late wife –someone who bears a resemblance but is undoubtedly her own person."

He stopped, unsure of her silence.

She could hear his heart beat accelerating and his palms cooling where he was holding her. She realized with a disbelieving gasp that he was nervous and literally in cold sweat.

She remained silent, carefully thinking over her words, "Not completely unlike."

He stilled, and she swore that his heart skipped a beat. "I know it's hard to believe that I wouldn't superimpose you—"

She pressed a finger to his soft lips. "I'd like to think that I can love you just as much as she did," she whispered, "and that you'll learn to love me just as much as you did her."

She could feel his lips turn up into a smile. She wrapped her smaller arms around his large frame.

They stood there together, in each other's embrace. Until her newly acquainted stranger broke the silence.

"Hello there, my name is Sesshoumaru. Can I get your number?"

Comments are welcome.

I will now be focusing on trying to get For the Ice King in Full Blossom done - but with my track record, we'll see how that goes. Fingers crossed.