Author's Note: I'm borrowing this idea if not the execution of it from Cypher3au's story Gene-Spliced Harry, and with a few tweaks it works in BtVS.
Disclaimer: I own neither Gene-Spliced Harry, Buffy the Vampire Slayer, Batman Beyond (which Redpriest17 says he borrowed the Splicer thing from – though I remember Gendo Giovanni using gene splicing on Pokemon before that came out.)
Xander looked at his father in stupefied fashion before shaking his head firmly and heading out before he decked him. "Idiot bastard told a science firm that I'd help them test some experimental medical procedure and I have to go see what the hell he tried to get me in for now."
Xander cursed fluently in several languages that he didn't even realize he knew as he continued down the street toward the GeneTech building where he paused to take in the exterior. The building was painted white and the glare would have been blinding if it hadn't been for the greenery arrayed around the place. He continued to curse as his feet resolutely moved him forward toward the entrance as his mind was still coming up with very interesting things to do by way of revenge versus the surly drunk known variously about town as either worthless or a prime supporter of their business depending on where he happened to be at the time.
The receptionist smiled brightly at Xander and shook her head at the various demonic profanities exiting his mouth, undoubtedly learned from researching with the Watcher at the high school library, how his father thought him stupid she didn't understand. Her bright chuckle as he described staking his father out to be bred to death by a chaos demon drew his eyes and she smiled broadly at him, her blue-gray eyes lit up and her straight white teeth sparkling in the incandescent light.
Xander himself was rendered speechless, the girl in front of him was obviously of some demonic breed or other – she not only understood his curses, she also had fox ears and a long fluffy tail he could see hanging behind her out the back of her chair through the glass desk and black claws were tapping gently at her keyboard. Aside from these obvious abnormalities she was in appearance a very beautiful woman in her early twenties or late teens with a lithe body and high firm breasts.
As he stopped speechlessly she smiled at him. "Welcome to GeneTech, I'm Amber and it's a pleasure to meet you Alexander," as the girl greeted him by name he looked almost as if someone had hit him with a brick as he gaped like a fish out of water.
"Xander … call me Xander," he dazedly replied as she stood and swayed over to him, gently taking his elbow.
"Your father scheduled your appointment for this afternoon and we have your medical records, it is as I said a pleasure to meet you, Xander."
"Are you… a kitsune?" Xander trailed off, unable to find a way to ask if the beautiful girl before him was a demon with any sort of subtlety.
"Please, do I look Japanese to you?" She grinned up at him impishly.
His brain reacting faster than his mental filter Xander replied, "No, but you're quite the fox."
Amber blushed prettily as she guided him down the hallway and toward an examination room. "The doctor will be with you shortly to answer all your questions before the procedure."
"Procedure?" Xander asked numbly as he took a seat in what looked to be a typical doctor's office, with the exception of the posters which featured humans with animal traits. Taking a closer look at one that appeared to be warning about the dangers of smoking it advised a one percent shark splice to prevent lung cancer.
"Splice?" he muttered his confusion plain on his face.
"Yes, splice," a tall man in a doctor's coat with more than a touch of cat about him said as he silently entered the room.
Xander opened his mouth and the man help up a hand. "Please no pussy comments, I've heard them all."
"I thought I saw, I say, I thought I saw a putty cat," Xander said in one of the worst Foghorn Leghorn imitations ever heard.
"Good lord, you took it as a challenge," the man groaned. "You know I can splice in a touch of parrot so you can at least sound like who you're trying to imitate, however I'm afraid there is no splice that can fix your horrible sense of humor."
"Splice?" Xander asked again with a raised eyebrow.
"Yes splice," the man replied. "Didn't you read the pamphlets we sent?"
"Never received any," Xander replied with a shrug.
"I sent them with your father so you'd know what we do here and to allay any fears you might have."
"Of course you did," Xander deadpanned, muttering curses under his breath again.
"You can curse fluently in several demonic languages, but you can't do a halfway decent Foghorn Leghorn imitation?"
"Cursing in foreign languages comes easily and how do you know about all of this?"
"I'm a Mymo'k. We naturally absorb DNA from local species which provides us many advantages, however it does not allow us to blend in very easily. One day one of us came up with the brilliant idea of figuring out how we absorb DNA and add it to our own and to provide the service for others. It serves several purposes. One; we can blend in to human society much better so we are not restricted to furry conventions and certain holidays, two we can provide the benefits of our talent for everyone and last but not least, we can become as rich as Nazis doing so," the doctor finished in a perfect mimic of Daffy Duck.
Xander just stared at him with shock written across his features.
"Most demons try and hide what they do."
"Most demons are idiots who want to run around killing people and causing chaos. My people just want to be able to relax and not worry about being hunted for our fur. Of course being able to do so while relaxing on a beach in the Caribbean and drinking champagne is just a bonus."
Xander thought about that for a moment and made a decision. "Xander Harris," he said offering a hand.
"Malcolm Brentwood, Doctor Malcolm Brentwood," the doctor replied doing a perfect Sean Connery.
"Okay, lay it all out for me. I know nothing about any of this. I was just told five minutes ago to come down here to be a guinea pig."
Malcolm rubbed the bridge of his nose, "Your father is one of the singularly most unhelpful examples of idiocy in ambulatory motion on two legs. Not only does he send a demon hunting teenager into a nest of unknown people for 'medical research' he sends one into a place that's full of demons masquerading as humans with obvious animal features. Its a wonder you haven't already hotfooted it to your little blond friend and her watcher, no offense intended Xander."
Xander shook his head ruefully, "That might be giving me more credit than I deserve doc, I've got the self preservation instincts of a lemming or so I've been told."
Malcolm again rubbed the bridge of his nose, and Xander grinned as he realized it was much like Giles' glasses polishing maneuver, "Teenagers …" he trailed off before looking at Xander sharply, "lemmings never leap over anything from which they cannot safely land so perhaps that was Watcher Giles' method of saying you may seem suicidal on the surface but in reality you are a survivor."
Xander paused thoughtfully, sticking his hands down into the pockets of his pants and leaning back against the exam table.
Seeing the young man was lost in thought Malcolm waited patiently for a few moments for him to work on the thoughts running through his head.
"So why me and what procedure?" Xander asked after a few moments of quiet contemplation.
"Let me answer that in reverse order. The procedure is called a shifting splice. You see quite a few people have non-human ancestry and don't know it. Normally that's not a big deal, as we can easily compensate for it, however there are species of demons with shape shifting abilities, even in the non full blood members. Of course that's really a misnomer, as there are no truly full blooded demons any more, but you get what I mean. As to why you? One of your great grandfathers was a Brachen demon. We chose you because of that fact. Provided everything goes well, I expect to be able to allow you to access your Brachen ancestry and a stable splice that will grant you all the benefits a normal splice would allow."
"As for why you in specific and not any of the other people with Brachen heritage we might have approached in this town; you have the most to gain and certainly would get the best use out of the procedure."
"What are the benefits of the splice?"
"As many and varied benefits as the animal kingdom allows, not to mention the various demonic kingdoms." Malcolm replied calmly with a smile.
Xander blinked, "You know doc that has some serious creep factor."
Malcolm chuckled, "I'm not planning to offer just anyone demonic upgrades, merely mentioning the possibility in all honesty to lay the cards on the table as it were."
Xander drummed his fingers on the table behind him thoughtfully as he gazed at the beige walls around him carefully, "What does having Brachen heritage mean for me hypothetically Dr. Brentwood? If you were to work your voodoo on it?"
"Ah, haven't read up too much on the peaceful races have you?"
"We don't tend to run into them much in our line of work."
"Point, but to answer your question even half Brachen demons have strength which compares favorably with that of a vampire; not just your average fledgling but a normal vampire. They're extremely flexible as most of their joints are double jointed; those with Brachen girlfriends definitely have no complaints if you get my drift. A sense of smell so acute that they can actually smell magic; I've heard it puts bloodhounds to shame. Lets just put it this way, you'll be able to tell what mood the girls are in a block and a half away."
Xander flushed, chuckling a bit; "I take it after a bit I wouldn't need a calendar?"
Malcolm smiled, "Exactly."
Xander grinned at the doctor, "Well, you have a lot more experience with this - what would you suggest?"
Xander yawned and stretched, before blinking blearily at his arms and then looking down at his chest. "Whoa, now I know how Spider Man must have felt when he first woke up. I got muscles in places I didn't know I had. " Xander then blinked, "Doc, are those spikes I have coming out of my hands?"
Amber chuckled throatily, "I take it the process was a definite success."
Xander's head whipped around to face her. "Why is everything in gray scale?"
"Don't worry, your color vision will come back in a half hour or so, your eyes just haven't adjusted to all the input yet," Amber replied as his nostrils flared and he sniffed several times.
Xander shook his head. "What's that smell?"
"Describe it for me," Amber replied, making notes on a clipboard as she looked over at him curiously.
"Musky, a little sweet … with a hint of salt, to tell you the truth it's making me a little hungry."
Amber grinned, looking down at his waist. "Among other things I see," she noted and he could see her cheeks darken.
"It just got stronger," Xander mused as her tail swayed a bit behind her.
"I think I'll send Doctor Brentwood in to see you now," she said as she swayed out the door swinging her hips with her tail moving flirtatiously.
Malcolm entered a moment later, dressed in the stereotypical white lab coat and eating a tuna sandwich. "How are you feeling Xander?"
Xander rubbed his forehead and yelped as he felt a warm diamond shaped object in the center of his forehead as well as stabbing the palm of his hand on a spike. "Great googly moogly! I feel like a porcupine!"
"Right now you look like one as well," Malcolm noted absently, "not to worry, you shifted into Brachen form due to the stress of the procedure. You can go back to normal at will."
Xander blinked looking himself over. "So... spikes on my face, whats this diamond shaped smooth thing in the center of my forehead?"
"I believe that is the Mohra demon anchor stone, though in an unexpected form … perhaps a genetic permutation due to the Brachen DNA that was also present."
Xander sniffed again hungrily as Malcolm approached him with a mirror. "Why do I have a Mohra demon anchor stone in my forehead?"
"Because Mohra blood is integral to the process and ensures that the subject; you in this case, survives. Though it seems we may have used a touch too much while compensating for the Brachen heritage. Well, we wont make that mistake again."
Xander winced. "So I'm going to have a diamond of some sort in my forehead?"
"Yes, but it looks quite fetching," Malcolm said, fluttering his eyelashes at him, "rather like Raven in the Teen Titans. Relax, girls love diamonds."
Xander shook his head ruefully. "Got another one of those sandwiches doc?" he asked as his stomach rumbled loudly.
"Several, I assumed you'd be hungry," Malcolm replied and rolled in a cart from outside the room with the requested sandwiches which Xander then tore into hungrily.
Between bites, Xander managed to ask, "So what does having the jewel mean for me exactly?"
"Best guess, without extensive testing which really isn't needed because it'll be rather obvious to you if things turn out the way I assume they will, you've absorbed more Mohra DNA than we had planned, meaning you'll require a higher salt intake... actually just normal fast food covers that. I swear the stuff most people eat is just begging for a heart attack. Anyway, I expect your... fluids will cause rapid cellular regeneration in anyone they come in contact with."
"Kissing some one's booboo will actually make it better, of course the more fluid the better with certain ones being much more potent than others."
"I ... have mixed emotions about my friends finding out about that."
"Well with the amount of injuries a slayer normally gets doing her normal day to day job... you may end up spending quite a bit of time kissing her in a variety of places, and of course slipping her tongue in this case would practically be mandatory."
"And then again mixed emotions is rather a strong term," Xander muttered, searching for more food.
Malcolm chuckled, "I thought you'd see it like that."
"So no other surprises, no suddenly switching genders based on water temp or anything?" Xander asked carefully before frowning and pulling out the front of his pants to make sure everything was still there. "Its got spikes!"
Malcolm chuckled, "Don't worry about it, you can resume human appearance … though there is one thing about that."
Xander glared over at him with a raised eyebrow, "What?" he asked warily.
"Much like a cat's those are a lot softer than they look, or there would never be part Brachen."
Xander frowned thoughtfully. "So like a permanent french tickler? And those cats on the receiving end don't
ever sound particularly happy."
"Kinky isn't just limited to the human race you know. Are there any other questions?"
"Yeah, how do I change back?"