Shirai is an Enervore created by the Elite Guardians, to protect the royal palace of Princess Celestia and Princess Luna. When Luna became Nightmare Moon, the result of the influence of the moon princess will turn her dark and feral. She's a neutral for the Villain League and Team Elite.
Before she was influence by Nightmare Moon, she once serve Princess Celestia and Luna as their guard of the royal palace. Till Luna grew bitter and she slowly turned dark. As she was Nightmare Moon's influence, Shirai acts like a mindless monster who starts feeding on Energon on other cybertronians. After Celestia banished her into the moon, Shirai still influecne by her dark magic, continuing her energon lore during the dark night. When she learned that the Mane 6 used the Elements of Harmony to defeat her and turned back into Luna, she was been connected by The Darkness.
In the Nightmare Arcee arc, during the events of Courage and Friends in Rise of the Guardians, Pitch (The Boogeyman) learned about The Darkness, Nightmare Moon, and Shirai. The results that he fought Celestia and Luna before, he uses darkness to turn all Daymares dark and become Nightmares. Luna was infested by Pitch's shadow shard in her hoove (didn't notice), she was becoming Nightmare Moon and Shirai. Shirai first met Arcee face to face, she was weak from the Guardians. She tell her that who order her to become a host of her, she said Pitch Black wants you to be her host. But Arcee refuse to accept her as a host, then Shirai make her do it. Arcee was unable to control her nightmare mode since Shirai's strong control. So she joined forces with Pitch Black and the Villain Ledge, leading an amy of Nightmares to destroy the children's believing the Guardians. Courage and Jack Darby tried to communicate with Arcee to fight it against Shirai, but she can't hear them under her control. Only Pitch could control her nightmare form, as Raven's elements Darkness, Optimus Prime warned her that she'll get corrupted too, but she didn't, Shirai grabbed her trying to turn her into a Nightmare. Then Bulkhead stopped her from being a Nightmare, till Pitch Black was defeated and Nightmare Arcee/Shirai got away. Ahsoka sences her that she still inside of her, worrying Courage. Until the arc finale Courage and Friends in Pinocchio and the Emperor of the Night, Pinocchio uses The Blue Fairy's power to scare off The Emperor of the Night, and Shirai can't stand her powers, and Jack tells her that you need to fight it, and remember about his dead partners, and himself. Arcee got free from Shirai's control, and resist, and she rejects her host body, and the darkness shards starts to fall off from her armor. Shirai helped Team Elite escape from the ship's destruction, and turned against the Villain Leauge. She watched Pinocchio defeats The Emperor, and scenting that he's still alive. She returned to Celestia and Luna to become a guard of the royal palace again.
- Shirai strongly resembles Technovore from Iron Man: Armored Adventures.