Magic Specializations and Personalities

Fire: The hot-blooded and aggressive attacker type is easily the most typical personality for Fire, but it's no straitjacket.  Instead of a raging inferno, you could instead emphasize the warmth of a fireplace.  That sort of Fire girl would be kind and protective of others, maybe even a bit motherly.  She also might be a leader among her group, as she's probably also reliable and good with people.  Regardless, she'd probably be the type whose magic rallies and encourages her friends, rather than the usual method of throwing fire around.  Still, she could surely do that when her anger is roused, such as by a teammate being KOed.

Metal: There are a number of qualities you could emphasize here.  One is as a very hardworking, driven magical girl, with steadfast beliefs and a powerful, albeit stubborn will.  Her fighting style might be straightforward attacking and defending with metal, strong but lacking in refinement.
Another option is a girl with a sharp attention to detail and a flexible, adaptive nature that prompts her to experiment with her powers and likely plan for a few contingencies.  Some might call her paranoid, but she's crafty and prepares for battles whenever she anticipates them.

(Later Metal write-up): I'd say one of the personality aspects that best fits is calculating and planning.  This could be a chronic worrier who preps gadgets for every scenario or a mastermind sort who knows exactly what she wants and how to make the tools.  Paradoxically, you might have a character who uses gadgets and tools to do things, as she's too unmotivated to put in the effort for something if she can just make a robot to do it for her, oftentimes repeatedly.

Darkness: Secrecy, wrath, and regret.  Darkness isn't a sociable element.  A girl who specializes in darkness may often seem quiet and keep to herself.  Even so, it would be a mistake to think that just because she doesn't often speak that she isn't listening.  Of course, while darkness might protect like a blanket, it can certainly smother just as easily.  The girl might come across as creepy or otherwise socially maladjusted.  Social isolation magnifies one's problems, so the bubbling sickness her power carries might harm others or the girl herself if she is left isolated and friendless.

Reinforcement: Altruism, protection, and wellness.  These three things fit well to reinforcement, but the manner in which a girl chooses to help others can easily differ.  One might be the at times overprotective sort of older sister who shields her younger teammates from harm.
Another might be the girl who is kind and helpful to a fault, though her magically enhanced body can easily break an attacking monster without ever touching a weapon.
As a third, the studious, hardworking girl dedicated to bettering herself and her peers.  She might alienate some people with her insistence on fitness and education over laziness and play, but she tries her best.

Stone: Metal and stone have their similarities, but a girl whose personality fits Stone likely has more rough edges.  A younger Stone girl might have a bit of an ego, as in her mind she's almost untouchable behind her walls and can shrug off what few hits manage to slip by her guard.  She might come across as a petulant child, but her bragging is mostly backed up by her talent and toughness.
Conversely, an older, more experienced Stone user has the wisdom to know her limits.  Having learned the hard way over the course of numerous broken bones, she realizes that a good defense isn't enough to win all her battles.  Not the type to be easily swayed, she thinks before she speaks and rarely jumps into fights she has no stake in.    When she does fight, instead of relying on walls and shields of earth to protect her, she reshapes the battlefield to entrap her foes and limit their movements.