Natalia Mitternacht first appeared in Robin #101. She played piano and moved everybody to tears except for Steph, who was in the rafters as Spoiler. Steph thought the music was terrible, and was nonplussed at everyone else's reaction, especially when she saw Tim crying.
Steph followed Natalia out of the play in time to see her attacked by a young man, and as Spoiler, she quickly knocked the attacker unconcious before he could do any real harm to Natalia. Natalia thanked Steph, but refused to give her name for Steph to give to the police.
Later, Tim and Steph grew suspicious about Natalia and staked out the playhouse, Steph having gotten over the nasty cold she'd had last time and Tim, on a suspicion, stuffed modeling clay up his nose. Steph was the one moved to tears this time when Natalia played, and Tim was not.
Steph followed Natalia again and the two of them discovered Natalia's apartment was on fire. Natalia rushed in to save her scientific files, and Steph rushed in after her, managing to save both Natalia, the files, and a fireman.
With Natalia at Steph's house, Tim and Steph managed to figure out that Natalia's body gave off pheromones that took away people's inhibitions, making them grandiose and emotional. This has had a traumatic effect on the man who attacked Natalia, Dylan Arthur Prescott, a recovering schizophrenic who had...well...un-recovered when he heard Natalia play music. He blamed her for the return of his hell.
Natalia was unaware of her power. She had been a brilliant young scientist until she had contracted her condition, which removed her own inhibitions so she studied unconventional things and dressed in medieval clothing.
Steph and Tim managed to save Natalia from the wrath of Dylan Prescott, though he sadly died while running away from one of his visions onto a train track. Steph and Tim sent Natalia to STAR Labs to have her condition checked out.
Natalia seemed to like Stephanie and referred to her as "little sister". Due to Natalia's power, Stephanie told her a lot of personal information, though it's unknown how much. When her nose was plugged, Stephanie was polite to her charge but obviously fairly weirded out. Natalia was extremely impressed with Steph's piano playing, while Stephanie thought Natalia's was terrible when her nose was plugged, but kept it to herself. Natalia ran away from Steph when Steph contacted Nat's mother, and when Stephanie found her again, she convinced the woman to go to STAR Labs by threatening to contact Nat's mother again.
This all happened in Robin #101 through Robin #105