Complete the letter analogy: C : I :: E : ?

The position value of the letter is squared and the answer is the letter standing at that new position. C is 3rd, and $3\times 3=9$, which is I. E is 5th, and $5\times 5=25$, which is Y. Doubling instead of squaring would have turned C into F, so that reading dies on the given pair. W, X and Z stand 23rd, 24th and 26th, the positions on either side of the 25th, and none of them is the square of 5.