gratuitous violence
She's not used to having a girl for a friend.
She doesn't really know how to act.
It's automatic, with Gray and Natsu and all those other idiots who she breathes for, because they're you know they're—boys. And it's said with this sort of derisiveness like they're dumb or something and—
(Okay. Okay, yeah. They're stupid.)
—it's not like she really means it, anyways. Because they'll look at her and Lucy and bags and bags of clothes and clothes and all those pretty things she couldn't get with anyone else, before, and they might look a little lost. Like they're not sure if they're supposed to hide all their wallets or help carry the bags or—or—blow something up.
They usually choose option C. Erza is not terribly surprised. Partly because she is also partly to blame for their enormous property damages.
But the point is, if Natsu does something stupid—er. Stupider than, you know, default mode, she can just sort of whack him and kick him around and yell a bit, and he'll get all fired up and then Gray will get all worked up too and maybe start taking his clothes off, (which Erza doesn't really notice anymore but it's still enough to make Lucy choke,) and they'll all start whaling on one another, no matter where they are—and then it's over.
Until, you know, Gray trips Natsu and Natsu sets Gray's butt on fire and Erza is forced to intervene yet again.
Lucy's different. Lucy's a girl. So is Erza, actually, but not the kind of girl Lucy is, with her sugar smiles and popping hair and flirty hips.
The good part is that Erza's never really wanted to whack Lucy, but at the same time, she's kind of terrified at how very, very new all of this is. So she keeps her distance, from Lucy and the seats she'll save for her.
So it's a slow process. It's subtler, with girls, you can't just—male bond. Male bonding is usually over some of the simpler and, frankly, more enjoyable things in life. (Namely meat and breasts and violent videogames and absolutely horrific 'art,' which is in all honesty code for 'soft core porn.')
They'll shop. And they'll go out for ice cream and get their nails done or watch that movie Erza has always, down to her toes secretly wanted to see, and it's nice because—because for once, she has something to indulge in, and a companion to do so with.
So when Lucy is actually evil enough to superglue Natsu and Gray's left feet together, Erza reaches out and smacks the back of Lucy's head before she realizes what she's doing and freezes up, suddenly petrified.
Lucy doesn't hit her back. She squeaks and pouts and rubs her head, but then grins and giggles and Natsu and Gray are subtly attempting to bite off her ankles in revenge, so Lucy scurries behind Erza and flies off with Happy, laughing like a crazy person at the death threats the two boys are screaming, though with no actual intention of following through.
Erza watches Lucy's skirt flutter in unapologetic fan service, and cracks a smile.
Maybe they weren't so different, really.
"PANTY SHOT!" Natsu shrieks, and Gray does some truly impressive acrobatics for a glimpse.
Erza kicks dirt in their gaping mouths and stomps huffily away.