Letitia was Jen's first imaginary friend, predating even her Ankulen. When she received the Ankulen, Tisha, as Letitia's name is often shortened to, was the first thing Jen brought to life.
As her oldest imaginary friend, Tisha was closer to Jen than anyone else. She knew all of Jen's deepest secrets, greatest fears. She wanted, more than anything, to make Jen happy, and, when she and Chris did the thing that got them in trouble, she honestly thought she was doing something that Jen would really like.
Her punishment was the opposite of Chris's. He was trapped in their dying world, she was trapped outside, forced to play the role of Jen's adopted sister, while Jen, quite frankly, hated her.
At the beginning of the book, Jen doesn't know why she hates Tisha so much. She suspects it to be because Tisha is so pretty and Jen is ... shall we say, plain in comparison. Jen doesn't know that Tisha's good looks and lovely name were her own fault - it never even occurs to her, despite the fact that she knows that she had an imaginary friend named Letitia prior to Tisha's adoption.
Tisha is gentle and sweet, and not particularly brave unless she has Chris or Jen to hide behind. She has pale skin, blue eyes and golden hair that falls to her feet in perfect waves (thus is why I want Rapunzel to play her.)
She is the Eve of Jen's world.
On another note, I don't have my own computer yet, but I just started chapter 21 in the notebook version - that means I'm almost done with the book!!!!!!!!!!!!!! I'm going to try to have it done by the end of April, we'll see how that goes. The last five chapters and epilogue, for some odd reason, were a quarter of the 1st draft, so they're meaty.
They're also the climatical and wind-up chapters, and, for some reason, I write those rather quickly. Chapter 19 marked 50,000 words, but I haven't counted 20 yet.
And I have decided, as extra motivation, I won't let myself buy that new computer until it's done. I shall leave now and return to the pencil and paper.