I'm in the camp that believes that these two characters were in a relationship towards the end of Order of the Phoenix, so here's a one-shot from then.
Remus Lupin loves his girlfriend. But he also loves a quiet half-hour catching up with the London Wizarding Journal. It's Sunday morning and he's on sofa at home (an ugly, threadbare thing. Remus doesn't know how many people have owned it before him) flicking through the newspaper. Thankfully the Journal is anti-Ministry, but it keeps it quiet. These days the articles are more academic than political. Remus is reading an editorial about goblin mines when there's a crash and a yelp of "Bugger!". Tonks is awake, then. Usually she's up much earlier than he is, but she's been on Order night duty all week. There's a series of thuds which is her coming down the stairs and then she pads into the living room.
"Morning, gorgeous," she says.
"Hello," Remus replies, keeping his eyes on his newspaper as he turns the page.
Tonks yawns, then notices: "Cool, you got Magikyan,"
She picks up a magazine and holds it up to him.
"Oh, it came free with the newspaper," Remus shrugs.
"You have your uses," she grins, and plops herself onto the sofa, feet in his lap.
"Dare I ask what it is?" Remus inquires, glancing up at her. Her hair's turned scarlet in the night and she's wearing his shirt from yesterday over her pyjama shorts. Flecks of ink dapple her right hand and there are a couple of bruises on her shins. She's bleary-eyed and rumbled and she looks so soft. She smells like a delicious cocktail of herself and him and sex and sleep. He can see the outline of her nipple against the breast pocket of his shirt. Remus' stomach growls, and it's nothing to do with not having had breakfast yet.
"It's a druid magazine,"
He narrows his eyes. "You're not a druid,"
"Druids are the best producers of folk- indie,"
Tonks swats him on the head with the magazine. "Yeah, you numpty. They look pretty cool, don't you reckon?"
She holds up a picture of a woman wearing a huge green dress, long black hair extensions and blue paint smeared over her face and arms.
"Umm, yeah. Really cool," Remus nods, flummoxed. His lack of knowledge about "anything that happened after the 1930s" is one of Tonks' favourite things to tease him about. Her love of "that showboating tuneless racket" music is one of Remus'. Tonks frequently asks him how Sirius turned out so cool while he's such a grandad.
"Think it'd be a good look for you," Tonks notes.
"Hmm, I don't think blue's my colour. Maybe purple,"
Of course she'd say pink. "Is this what it's like being you?" Remus asks, "Thinking of different colours to turn yourself?". He used to be wary of asking her about being a Metamorphmagus. After all, he's used to endless unwelcome questions about his own transformations. But Tonks would talk about morphing all day if she could, and she relishes showing off with it. Remus' favourite thing is that she uses it for fun. Hermione Granger is nearly seventeen, she's the cleverest teenager Remus has ever met, she's the serious and stern one of the kids' little family- but Tonks' pig nose gets her giggling like a little girl. He loves that Tonks can make people happy with something so simple and silly. Come to think of it he's often been the Hermione of his friends, but joking is so easy with Tonks. She has turned everything he thought he knew on its head.
"Pretty much, yeah. Choose one for me now?" she asks.
"You'll match me," he points out.
"Your hair's not grey, Remus," Tonks answers defensively, "It's brown,"
"That's very kind of you," he says, patting her knee.
"I mean it. But when you do go grey I'll be totally digging the silver fox look". She screws her face up and the red fades out of her hair, leaving it a dull silvery colour. Then it grows upwards, curling into an old-lady perm.
"How's that?" she asks.
"Beautiful," Remus grins.
"Yeah, I've heard all blokes dream of an old lady sitting on their sofa wearing their shirt,"
"Teenage fantasy totally realised," he jokes, exaggerating the 'totally' to mimic her. Tonks jabs him in the ribs with her foot.
"Do you have anything for breakfast?" she asks.
"Umm….bread. Maybe some raisins too?"
"How have you not starved to death by now?"
"No, I'm kidding you. There's eggs in the fridge and a box of cereal in the cupboard,"
Tonks' reply is gets lost in another yawn, and she stretches as she gets to her feet. Remus cocks his head to watch her, admiring how arch of her back leads into her neck, and down to the roundness of her breasts. He had forgotten, is his long periods of solitude, the plush physicality of women. The mingling of hot breath, the friction of hips on hips, the sensation of burrowing his face into- well, anyway. Remus fumbles with the newspaper to distract himself.
"Do you have orange juice?" Tonks calls as she walks into the kitchen.
"What do you drink at breakfast, then?"
Tonks leans out of the kitchen to goggle at him. "Remus, are you a monk?"
The Sirius in his head drawls, a monk wouldn't have done what I did with you last night, but Remus shrugs and says, "There's no point buying anything to drink that isn't Butterbeer or wine. Or hot chocolate. Although now I think about it," he remembers, "James had a phase of pouring Butterbeer on his cereal,"
"How did that work out for him?"
"Alright, actually. I tried it myself a few times and it wasn't too bad. He might have been on to something,"
He likes telling her about James and rest of the Marauders when they were young. Remus talks to Sirius about them sometimes but the years of guilt, anger and betrayal have battered both their memories. Whereas with Tonks it's just sharing anecdotes about a bunch of silly boys. It feels lighter.
"You're bonkers," she tells him.
"Brave words coming from you," he says, and clicks his tongue (then catches himself- since when was he the sort of man who clicked his tongue? The answer is, of course, since her).
Tonks starts to retort but her words are interrupted by a clatter, which Remus takes to mean that she's attempting to make breakfast. Then there's a clang followed by a mutter of, "Dammit".
Remus smiles to himself, exasperatedly, affectionately, and turns back to his newspaper.