It's always useful having a fairy around when attempting to wrangle a demon. Erza requips her Heaven's Wheel armour, with its silver pleats and many swords, and braces herself in front of Mirajane in case the worst happens.
When she transforms, Mirajane tends to lose a piece of herself. The anger, the power, how can she deny herself something that feels so good? Erza understands, she supposes, with her supply of armour and swords allays ready at a moment's notice. The strength magic gives them cannot be matched by anything.
Admittedly the fear of losing herself has always been in the back of Mirajane's mind. When she or Elfman transform, she always wonders if this will be their last moment spent as members of Fairy Tail.
Between heartbeats, Mirajane takes Erza's hands in her own and whispers her lover's name. If this is it, the moment where she loses all control over her demon form, at least she died with a prayer on her lips.
She tells herself it's only magic, yet knowing she could hurt someone, or worse, kill them, has always left Mirajane on the back foot when it comes to using her natural talents.
"Look at me," Erza commands. She isn't quite sure if Mirajane hears her or not, given how fast she transforms. Her unassured smile turns cruel, a set of fine leather wings unfold in all their hellish majesty, and in the next moment her eyes soften. "You did it."
"Yes, I did."