I do have these lego. In fact, these sets were not only available in Japan. At that time, you can find them easily in certain asian countries. Harry Potter lego are not very popular in Asia and I think maybe it was just a good idea for LEGO company to consume extra minifigures.
I know some of them come from the promotion of toy stores if you buy lego products over certain amount ; other come as free bonus once you go watching the first round HP movie.
There are total 17 HP minifigures in 4 differnet boxes. You always know what you got once ckecking the pictures of the minifigures' faces on both side of each box.
1st
Draco Malfoy
Professor Severus Snape
Professor Albus Dumbledore
Ginny Weasley
2nd
Crabbe
Ron Weasley
Rubeus Hagrid
Mr. Dursley
3th
Professor Gilderoy Lockhart
Madam Hooch
Harry Potter
Lucius Malfoy
4th
Tom Riddle
Professor Minerva McGonagall
Dobby
Goyle
Herminne Granger
If you are interested, I can post more detailed pictures later. However, I really don't think they worth you spending lots of money on them unless you are a big fan of HP. All you can get are only 17 Harry Potter minifigures, some weapons, 14 capes, and four boxes written with "Free gift only, not for sales" in English and Chinese.