I don't like Diamond. I know many people do, but to me he's just a too weird character. What's with the eating all the time? I know they're supposed to be funny, but it's a bit too unrealistic. I can't feel for him. All the other characters have background stories that makes you understand who they are... It could be that I haven't read far enough into D/P to appreciate him yet (I've only read as far as the Mangafox scans cover) but I really hope Berlitz fall in love with Pearl if any of the two.
Ruby is a cool character, but a little too exaggerated. At first, he was balanced, but now he's as fighty and brave as Saph and he's sort of... he's lost his interesting weaknesses. He's a little too cool.
And Emerald is annoying. It got a little better once I understood why he was looking like he did and all. Plus, he's friends with Crys :D But still, too weird a character.