If you intend to preserve Ender's name, family, personality, etc. as you suggest, then you will run a far higher likelihood of violating copyright law than if you changed the name and/or the character's qualities.
So a lot of fanfic does technically violate copyright law, but oftentimes the original author doesn't care because the derivative writing isn't taking away any $$ from the original work--indeed, fanfic in many cases creates more interest in the original. There are a few writers out there who come after fanfic writers really hard, but most don't care or don't bother. And a few are actually encouraging. You can check online about whoever wrote Ender's Game and see what, if anything, he's posted about fanfic.
Now I'm not saying that you're writing fanfic; that's just the most commonly seen way that writers use another writer's characters in a novel or other piece of fiction.
Bottom line: don't worry about it too much unless you get a letter from the publisher or author's attorney telling you to stop. If you do get such a letter, you'll have to pull whatever you've written from the web. (They have to send you such a letter before suing you under the federal copyright law.)
You would also have to deal with the potential copyright issue if you ever get published. Then you or your publisher will have to get permission from the creator of the character before publication.
