When you add a category to a page, please make sure that there is information on that page that proves it fits. For example, if you're going to add the Deceased category, there needs to be a sentence that describes how the character died.
A lot of people are adding categories for two reasons:
They are familiar with the story so they know the category fits.
They are adding categories just to add categories and it doesn't matter if the categories are true or not.
For the first, that may be correct but how is anyone else supposed to know that the category is true? Putting a category on a page without proof means we're just supposed to accept it's true.
The second is a bigger problem. It leads to pages that have more categories on them than anything else, such as one sentence that says "Heinz Doofenshmirtz is the main antagonist of Phineas and Ferb", followed by eighty-seven categories. When those categories are clearly wrong and the person has been adding wrong categories for a long period of time, that becomes vandalism.
Categories really should be used to help organize the page, not as a substitute for writing sentences that provide information about the character. Too many people are click happy with the Add Category button. Please continue to help out by taking the time to say things about the character and not just dumping them in categories or the Infobox.