Why the "Parasite" didn't want Dresden to know about its true nature?

Question

At the beginning of "Cold Days" Dresden finds out that there is something living in his skull, that gives him enormous headaches:

My head was starting to ache again. Dammit, this was all I needed. Over the past few years, my headaches had grown steadily worse, to the point where sometimes they all but knocked me unconscious. [...]Demonreach growled. In all capital letters. And the headache vanished. Whoa,” I breathed. “Uh . . . what did you just do?” “I WARNED IT.” I blinked several times. “You . . . warned away my headache?” “THE CREATURE CAUSING IT. THE PARASITE.”

But when Harry tries to find out more about the nature of the beast, its stonewalled:

That’s right. Right here in this chamber, the last time I’d been here, either Mab or Demonreach had said something about the division of labor keeping my body alive while the rest of me was elsewhere. They’d said that the parasite kept my heart running. I glowered at Demonreach and said, “Tell me about this parasite.” “I WILL NOT.” I made an exasperated sound. “Why not?” “IT BARGAINED.” “With what?” “YOUR LIFE, WARDEN.” I thought about that one for a few seconds. “Wait. . . . You needed its help to save me? And its price was that you don’t tell me about it?” “INDEED.”

So at this stage reader (and Dresden) can suspect that this thing is indeed something malevolent, akin to alien creature or demonic entity.

But then, in "Skin Game" it is finally revealed that

It is in fact a spiritual child of Harry and the imprinted copy of Lasciel (or Lash as he called her) that was created in the moment when she sacrificed herself to save Dresden.

So why such secrecy (other than to keep the reader guessing)? Everyone who could somehow benefit from this knowledge (Mab or entity's "mother") seems to know its nature, so why holding this information from Harry?

Answer 1

We don't really know much about what the "parasite" wants; the end of Skin Game implies that we're going to learn a lot more about it in the next novel.

However, it's important to note what Mab told Harry about the parasite: that it's presence in Harry's head is killing him. The spirit is growing inside Harry's brain, and it will eventually overwhelm him and kill him if it can't be taken out. Mab also says this:

"The Parasite," Mab said, "when it kills you and emerges, it will seek out everyone you know, everyone you love and it will utterly destroy them, starting with one child in particular." She was talking about Maggie, my daughter. "She’s out of this," I said in a whisper. "She’s protected." "Not from this," Mab said, her tone remote. "Not from a being created of your own essence just as she is. Your death will bring a deadly creature into the world, my Knight. One who knows all you know, your allies, lovers, family."

Mab can't lie, so there must be some kind of truth to what she was saying. Or, at least, Mab believes there is truth to it. Most likely, the spirit made a deal with Mab and Demonreach not to tell Harry anything that would entice Harry to try and destroy the Parasite before that happened. Alternatively, maybe it was afraid that knowing what it was would distract Harry and get him hurt, releasing it into the world prematurely.

(Why all these bad things are supposedly going to happen, and whether Molly has managed to side-step them, are things I assume we'll learn more about in the next book.)

Answer 2

We've only had two scenes where the parasite has spoken, one where it mostly said "I'm sorry", and another where it was 90% asleep. Safe to say any answer on her intentions would be speculative.

For my personal reasoning, it's important to remember that although sentient and bursting with knowledge even before being "born", the spirit is still a child (who, as pointed out in a comment by Shamshiel, does have a name, and per this AMA that name is:)

Bonea, formally, or Bonnie, in Dresden's usual style

What's the first thing a child does when it causes a mess? The kid tries to hide it. Or hide themselves.

His subconcious finally introduces them in the first scene mentioned, but only after she's been bound up by Mab's icy earring. I think she was hiding as much as she could before that. Not knowing what could happen once Dresden was aware of her, possibly knowing the last being to occupy that particular gray matter didn't end well, possibly not even aware that it needed to be "born" to stop being a pain in his head, its first reaction would be to hide in fear. That's my 2 cents anyway.