* Have vibrant character traits that play into the story:
They must have a decent set of positive and negative traits that make them interesting. They can develop over time, so long as the show doesn't forget their base personality (character development ISN'T turning someone into a Purity Sue). For their flaws they must be developed by the plot and they must be called out or punished. For their positive traits, they must have positive effect on things to be seen as such and recognised. Actions have consequences must be a recurring plot line.
* Must be effective to the plot:
While I don't mind filler characters, a character that is designed to have a big part in the plot must have a meaningful role. If they are a hero, they must accomplish things and prove their competence. If they are villains, they must foil the heroes and give them enough threat to prove said competence. A character that is forced into the plot meaninglessly without accomplishing much will get on the audiences' nerves and be considered superfluous or even harmful to the plot. Please note that a character who has good or bad things HAPPEN TO THEM isn't the same as this quality.
*Chemistry with the rest of the show:
The character must have the ability to bounce off of at least one or two other characters in a developed way. This is more than just a few running gags. This interaction must help develop on both sides' characterisation and make them more interesting and developed. If they are friends they must show blatant care and bond with each other. If they are rivals or enemies their antagonism must be developed. Avoid forced or shallow relationships.
* Must have SOME gratification in watching their role:
A bit of a personal preference, but I tend to prefer characters who have worked hard enough to make some sort of difference in the plot. The 'winners vs losers' plot is funny in doses, but it dumbs down the plot after a while. A winner that always wins without having to even try will not seem competent or sympathetic, while a loser that never succeeds in progressing or having effect on the plot will seem static (and border on depressing in time). I want to see a character work and strive to accomplish something. PROVE THEIR WORTH. I want someone to root for, and it doesn't necessarily have to be the same person over and over.
The more I think about these, the more I think XY has failed in a lot of regards to them.