If you're wondering how Naminé adds links to ones memories well, she uses her sketchbook.

Many call her an artist because she constantly draws in her sketchbook. The sketches she makes are usually the new memory links that she would add to one's heart making you believe that the certain event she's made actually happened.

Since she can only control Sora's memories and those around him, just taking apart Sora's own, will lead to unchaining his own friends' memories. She cannot destroy your memories, she can only unchain the links and re-arrange them. The remaining pieces that she took out will sink into the darkness of your heart, but that doesn't mean you can't retrieve them again.

Naminé is proclaimed a witch because that is what DiZ calls her. She has unusual powers to manipulate memories, and even objects that revolve around Sora's most important memories.