Soul
Happen all the time. He'll just grab you and start kissing you anytime that Maka really pisses him off.
Black star
He's super goofy about them, until he gets really into it, then it's just fun.
Kid
The first few, he tried to be symmetrical... It was just weird. When you started refusing them, he got the message... Now they can get pretty heated if you don't pull him back a bit.
Crona/Ragnorak
You X Ragnorak happens all the time. Your lips always end up bruised. When he's done, he always needs to leave so he can 'take care' of his problem. Usually, Crona will come up and peck your lips before cuddling with you, but sometimes, if Ragnorak leaves you a panting, blushing mess, he'll get a bit bolder. He usually cradles you in his arms as he kisses your lips, even daring to slip his tongue in. Ragnorak usually walks in on these. He'll smirk as he strides over to you two, kissing Crona's lips while you start kissing his neck or collarbone. Crona is always turned into a panting, trembling mess trapped between the two of you.
Excalibur
Not as rare as you think... They normally happen after one of his 'fool' fits. He thinks he hurt your feeling so he'll pull you close and show you how much he loves you.
Stein
If he has a really bad day, he'll pull you close. He'll kiss your lips ever so softly before deepening the kiss and pinning you to the nearest wall.
Spirit
They're tackle kisses that ever so quickly become heated.
Medu
His favorite thing in the world. He hasn't asked for anything more out of the relationship than your kisses.
Mifune
Most often, the two of you will be guarding the house when he'll call you over to him. He'll pull you close, making you straddle his hips. You start to kiss him as he pulls you flush against himself.