The point of copyright law is primarily to protect the financial interests of the original author. If you copied long sections of "Ender's Game" into your book, to the point that a court concludes that someone might buy your book INSTEAD OF buying "Ender's Game", you would lose a copyright lawsuit. If you just copy a few snippets here and there, it would probably be considered "fair use". If you just referred to the character without copying any words from the original besides his name, that would definitely not be copyright violation, because you haven't copied anything.
It might be trademark violation, which is a whole different issue. If Mr Card or his publisher believes you are appropriating their trademark to take advantage of their good name, they could sue you for trademark violation. In this case there would presumably be little question that you are indeed deliberately copying there trademark. So the only question for a court would be whether you had done them any harm.
Regardless of the legalities, you might seriously consider what you are trying to do from a literary and artistic point of view.
Using someone else's character is generally seen as being lazy or uncreative. Readers expect you to invent your own characters.
If you just make a quick reference to someone else's character for a joke or a comparison, like "he's as strong as Superman!", you'd almost surely get away with it legally and artistically. I recall a novel I read once that had a brief section where the hero runs into a strange space-time anomaly and heroes from 3 or 4 other's people books show up. They all quickly leave and he gets back to his own characters. Personally I thought it was awkward. I suppose some readers thought it was cute. But drag that sort of thing out much and it just makes your story look like fan fiction rather than an original work.
* Update *
The name of a character, or basic ideas about a character like what sort of person he is, are not protected by copyright.
From a US Copyright office publication:
Can I copyright the name of my band?
No. Names are not protected by copyright law. Some names may be protected under trademark law. Contact the U.S. Patent & Trademark Office, [email protected] or see Circular 34 "Copyright Protection Not Available for Names, Titles, or Short Phrases".
How do I copyright a name, title, slogan, or logo?
Copyright does not protect names, titles, slogans, or short phrases. In some cases, these things may be protected as trademarks. Contact the U.S. Patent & Trademark Office, [email protected] or see Circular 33, for further information. However, copyright protection may be available for logo artwork that contains sufficient authorship. In some circumstances, an artistic logo may also be protected as a trademark.
How do I protect my idea?
Copyright does not protect ideas, concepts, systems, or methods of doing something. You may express your ideas in writing or drawings and claim copyright in your description, but be aware that copyright will not protect the idea itself as revealed in your written or artistic work.