Her character, is very private and keeps much about herself secret. The producers even apparently kept most of it secret from Olivia as well.

[That] she may or may not have Huntington’s, that was something I found out a little earlier than everyone else, because I needed to know why she was so guarded.

I went to David Shore and said, “I know you guys keep everything secret. You’re so tight-lipped and I respect that. But I need to know early on, what she’s comfortable with and what strikes a cord with her.” And he trusted me and told me and I didn’t tell a soul, even though it was difficult. But that was good; it was getting me in character. And once I knew, it really shaped her for me and it made me have a lot of empathy and respect for her. And as I continue to learn about her, the same thing is happening. I’m growing to love her more and more. But they’re very good about keeping things very secret on House, even from us. But they respect us and if we need to know something, they’re cool about letting us know.

When I asked her about the nickname her character has and about the characters real name, she said that she did indeed know the name and loves it, but is not sure we’ll find out anytime soon.

I thought they were going to reveal it awhile ago, and I think it’s great that the producers made this dramatic choice not to reveal it when they had planned to. And they actually tried to hide it, it disappeared from the call sheets. It disappeared from around the set. I think they’re not putting too much value on the secrecy of a name, but they like the fact that it is established this sort of inside joke between House and Thirteen. He could easily look at her file and see her name, but he chooses to keep it up.
I think they have an informal relationship. She has almost from the beginning called him just House, as they all do. And it’s almost like the formality of Dr. This and Dr. That has been taken away. And so using her number still at this point when he knows her pretty intimately at this point is a way of acknowledging the joke of her attempt to keep everything tightly held to her chest. And I think that it’s a sort of tongue-in-cheek way of saying, “Okay, you can try to keep it in. I won’t use your name, but I’m going to find out everything else.”

And the rest of the team uses it, I think, as kind of a joke. They could also find out her name if they really wanted to. But at this point, it’s kind of remembering the beginning when she was so tough and still tries to be, but she’s breaking down a little bit.

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