It was a good movie, but didn’t you find the relationship between the two lead characters a little disturbing?
You know her falling in love with him is quite plausible in that situation. He's the first adult ever to be nice to her. I give kudos to Besson for having Leon keep it a father/daughter, mentor/student, friend relationship. If you watch the version with deleted scenes, he refuses her advances.
Of course what disturbed most people was a 12 year-old girl learning to become an assassin, and taking part in hits.