[Decimus]

Why are Chinese fictional characters in Japanese media (anime, manga, games, etc.) frequently portrayed with a habit of attaching -ある to the end of their sentences? (when speaking Japanese) Is it due to the 'broken' Japanese spoken in the formerly-occupied Manchuria, the mish-mash of Japanese and Min Nan spoken in modern day Taiwan, possible broken Japanese spoken by Chinese immigrants or Chinese foreign students, the Chinese interjection and sentence intensifier ”阿” (which is analogous to the Japanese 「あっ」 and　「よ」), a combination of the above, or something a writer cooked up for his work?

I don't find the stereotype to be offensive, much less racist. I'm actually think that it's funny and somewhat endearing. I'm still curious though.

Ironically, when I was first learning Japanese, I had a brief habit of using 「である」 instead of 「です」 since I started off by trying to read "formal" documents. (which was admittedly a bad idea.) That said, I think it's just a coincidence.