Every writer falls in love with their characters. We love them. They may irritate us from time to time, but we love them. Here are some tips to make sure that love hasn't got you viewing your creations through rose-coloured glasses:
They say the wrong thing which causes hurt feelings or misunderstandings.
They have a physical flaw (even if it's only noticeable to them).
They're not a stereotype. (harder to realize than you'd think)
They're not successful at every venture.
They react uniquely to each situation you throw at them.
You've given them something to overcome (could be small, could be huge, but there's something).
There's something about each character that drives you a wee bit batty.
They don't remind you of any character you've ever read. (be honest)
Their egos make them make mistakes.
Unless you've written Satan's evil twin Skippy, (and maybe even then) each of them has at least one redeeming quality.