There is a word for characters like this.

Mary-Sue.

Avoid Mary-Sueage at all costs. If you cannot avoid this, (you know, making a charater beautiful and nice and shy and perfect), then please, don't write. Just don't.

Some tips:

1.) Give your character at least one big flaw in her appearance. Hermione, for example, has bushy hair and buck-teeth. We like her like that.

2.) Give her one big flaw in her personality (as well as several small ones--nobody has perfect, or even almost perfect personalities). Maybe he/she's mean and judgemental, but something happens to make him or her improve in the end. Or not improve--either works.

3.) If JK has already made a family, don't add to it. Hermione doesn't need a sister, Harry doesn't need a twin brother, and Dumbledore doesn't need a daughter. This definitely applies to Fleur. If you're going to make an OC (original character), make a new family to go with them.