In my personal Mentalist fantasy world they've been dating since Red Hot, so this takes place in mid- Season Four.
Disclaimer: I do not, sadly, own The Mentalist.
Neither of them liked Christmas much, not really. She usually evaded her brothers' invitations to visit and instead holed up in her apartment on December 24th with a pint of Cherry Garcia and "It's a Wonderful Life" on DVD. He would either fly off to the Bahamas with the supermodel du jour and feel guilty for leaving his mother all alone, or he would spend it with her at the retirement community in Florida, regretting the decision after the first thirty minutes of criticism and reproach.
Somehow everything felt different this year. Walter couldn't have explained what made him purchase the tree, nor what made him forego handing it over to his interior designer for decorating. Instead he opted to trim it himself with sparkly red and green balls and a variety of ornaments he'd found on the Internet late at night, including a miniature football stamped with the Chicago Bears logo.
When she knocked on his door at 10 pm on Christmas Eve, he was already beginning to feel a serious knot of unease in his stomach. This wasn't him, the tree glittering in the corner, the carols on the CD he'd just bought wafting from his state-of –the –art speakers, the smell of the hot mulled cider he'd earlier put on the stove to simmer. He couldn't say what weird elvish presence had taken over his mind, but clearly this was wrong. He wondered for a moment if it was too late to fly off to the Bahamas. Images of his usual hotel flashed through his mind- the white sand beach, the hot tub- but her knock came again and there was nothing to do but answer the door. He could always laugh it off, claim that his housekeeper had done it on a whim and plead innocence of any potentially embarrassing Christmas spirit.
After a year of dating, he'd grown familiar with her moods. When she and the team had solved a case, her green eyes sparkled and she moved with the alacrity of a curious chickadee. On bad days, when they got there too late or when she'd had to stay most of the night working on paperwork or when worry over Jane's mental health was gnawing at her more than usual, her eyes dulled to a stormy-ocean grey and the corners of her mouth drooped in that sad-puppet way that always made him want to kiss them back into a smile. Today had clearly been a bad day. Dark circles ringed her eyes and her lips were set in a weary slouch.
As he stood in the open door she forced a smile, stood on her tiptoes to kiss him apologetically. "I'm afraid I'm not going to be very good company tonight. Maybe I should just go home…"
'What happened?"
She waved a dismissive hand in the air, a gesture that encompassed any number of possible Jane/homicide/bureaucracy-related woes.
"Doesn't matter. I'm just not sure I'm really up for any- what's that?
She crossed to the tree and examined it, the balls, the tinsel, the flashing lights. She reached out a finger to touch the little football.
"You- got a Christmas tree?"
Now was the time to mention his housekeeper, but for some reason he couldn't quite make the lie pass his lips.
"I usually don't-it just felt like the thing to do this year." He shrugged. "You never know- maybe Santa will notice how good I've been recently and drop off something interesting as a reward."
He was relieved to note that her eyes were beginning to shine again.
"Excuse me, but exactly how do you figure you've been good this year? I can't say I've seen a lot of evidence."
He rolled his eyes.
"Teresa, give me a little credit. I have engaged in precisely no industrial espionage in the last year. It's been over twelve months since I've been a suspect in any of your murder investigations. I even called my mother this afternoon to wish her a merry Christmas."
"And how did that go for you?"
That was not a subject he wished to discuss right now, not with her eyes glowing and her lips curving into a quizzically luscious smirk.
"Irrelevant. Point is, I did it. So I think I'm entitled to a little Christmas reward."
"Did you have anything in particular in mind?" she asked huskily, a touch of flirtation warming her smile.
He pondered for a moment.
"Well, I've been trying to take over Microsoft for a while now, but that may be beyond even Santa- ow!"
The smack on the bicep she gave him stung, just a little. For such a tiny woman, she was freakishly strong. For some reason, it never failed to turn him on.
"Anything else?"
She had moved in close to him, close enough that he could smell the cinnamon fragrance that never failed to trigger the arousal in his body. He associated it with her so strongly now that it caused him awkward moments in bakeries sometimes. Wrapping his arms tight around her, he buried his face in the dark silk of her hair.
"You, OK?" he murmured. "Better answer?"
He felt her smile against his shoulder.
"I have to confess, I do like being ranked above Microsoft."
She pulled slightly away and looked up at him, her eyes catching and holding his. "I love the tree. I'm so glad you got it. I haven't had one since I was a kid. My mother used to do that- we'd get home from church on Christmas Eve and there it would be, all decorated and shiny and ready for Christmas morning…I missed that."
She took his hand, small cool fingers interlacing with his larger warm ones.
"Speaking of Christmas morning, we should probably get to bed soon so Santa can come."
Walter resisted the impulse to observe that if he had anything to do with it, Santa wouldn't be the only one. Instead he happily followed her toward the stairs that led to his bedroom, glancing back at the winking tree and thinking that perhaps there was something to this whole Christmas business after all.