This is one that I've had several different ideas about, but they're all pretty similar.
One variation of the idea was that she mainly manifested herself as a hologram, though she was a real person. She could control when people could actually feel her, and when she would pass through things. She was very knowledgeable about technology and engineering, though no one really known anything about the real her. No one had seen her, and most didn't even know if she was a real person.
Another idea was that she could enter into electronics and move around inside them. She could move from one device to another if they were in the same room. If she forms a connection between the two while inside one of them, she can move between them when they're further apart. The more she's done it with those devices, the easier it gets, and the further she can go.
And last, I had the idea that she could be an artificial intelligence, like Jarvis or Ultron from Marvel, but with a body like the one from Agents of Shield (who I can't remember the name of).