Natashia "Tashi" Repina was a young 13-old mutant from Carnelia. One day — due to an accident caused by her powers — her parents died. Tashi escaped and hid in a warehouse. Lonely and scared, she fell asleep and was trapped in her own nightmares unable to wake up. She was discovered by several guards, but, affected by her power, they were twisted into monsters which only fueled her fear.
The situation alarmed mutants on Krakoa, so the New Mutants went to Carnelia. Karma tried to contact Tashi with telepathy, which backfired and dragged her and other team members into the girl's nightmares. Eventually, Wildside managed to get close enough to Tashi to affect her with his powers and transform her nightmares into happy dreams. As a result, she and everyone else were freed.
The New Mutants brought Tashi to Krakoa, where Forge created a special containment reservoir for her. Dani Moonstar tasked Maxime and Manon with taking care of Tashi. Manon proposed helping Tashi feel better by removing her memories about causing the death of her parents. Moonstar refused because they can't remake people, even if they think it would help them. She told them to only help Tashi stay calm until they found a way to help her control her powers.
Later, she asked Dani to be her partner in the Crucible so that she could regain her old appearance, but Dani refused her request. She was instead comforted by Anole, a fellow non-passing mutant. Under the guidance of Amahl Farouk, Cosmar and her friends began experimenting with the corpses in the Boneyard, so as to transfer their consciousness to one of the bodies in order for them to look normal. They were interrupted by Scout, who was concerned by their actions.
On the night of the Hellfire Gala, they found Scout dead while exploring Planet Arakko. Deciding they couldn't trust the adults after their recent secretive and unhelpful behavior, and not wanting to get Farouk caught up in something he'd probably get blamed for, they decided to get Gabby resurrected themselves by going to Arbor Magna to use Cerebro and put a copy of her mind back into her body (which was being controlled by No-Girl.) When they got there, No-Girl tried to mind control Tempus but they were confronted by Wolfsbane in wolf form.
Eventually, the New Mutants took her to see Masque, who was using his powers as a plastic surgeon at the Moira MacTaggert Memorial Public Hospital in Madripoor. Masque helped Tashi restore her body to normal proportions, with only the purple skin and yellow right eye showing as part of her mutation.
Oneiric Reality Warping: Tashi's dreams affect reality around her. Her dreams form a sphere around her and everyone who makes contact with it — either physically or through telepathy — will be dragged and trapped inside. They will be connected in one neural network with Tashi and be used as a power source for the sphere. People trapped inside are affected in different ways, some can be twisted into horrendous monsters, while some can see their own fears or dreams.
- Krakoan Gateways
- Given her powers, "Cosmar" is likely supposed to be a transliteration of "Кошмар" (Russian word meaning "nightmare"), although "Koshmar" would have been more correct.