Possibly the letters with no lines of symmetry.
y....all others come just two letters after every vovels..
K - The pattern is to skip one letter of the alphabet in between each - between D and F there would usually be a "D", between F and H a "G" and so on.
It's the letter D of course! Haven't you heard of Jason D?!
Duck Tales was a TV cartoon series in 1987-1990. It begins with the letter D.
The next letter is D. The pattern is every fourth letter. Ie. a b c D e f g H i j k L m n o P and so on...