What are the different services for ringtones or other music download
for mobile phone in Japan?
How much songs do they offer and what type of songs (westerns, local
japanese or both)?
How much does it cost, per download or monthly?
How many download does an average user do? What is the average user
(age, gender, location...)
What kinds of phone are usually used for mobile music (Brands, models)
and what are the characteristics of the music (sampled, like MP3 or
not like, MIDI, how many channels, sizes, and lengths).

I can comment as a user in Japan.

There are many many sites, and a few listed companies that run them.
Western, Japanese (pop and traditional) music is available, both as
electronic tones, and as recorded music clips.

I pay 300 yen (approx $3) for 15 downloads a month - I subscribe to an
exclusively western site, which is available in English. I check the
site once a week or so (when I'm bored in a taxi) for new releases, or
intensively whenever I upgrade my handset (can't take them with you
when you upgrade).

Average user trends younger obviously.

Hope this was helpful - I realise its a long time since you asked the question.