Question about non permanent tax resident and foreign sourced income

I think I have a pretty good grasp on the topics of income tax for non permanent tax resident, and whether an income is japan source or foreign source depends on the physical location of the person when the work is performed.

However what I would like to understand is , in a more unconventional manner, how are income and tax treated in the following scenario?

1. An individual is a non permanent tax resident in Japan
2. The individual receives monthly salary from a Japanese entity, pays income tax in Japan
3. The individual also receives additional fees (retainer or consultancy fees, does it make a difference) each month from a foreign entity paid into a foreign bank account
4. The individual is outside of Japan for 1-2 weeks every month/ quarter (does the length of stay and frequency outside of Japan matter?) to perform the work relating to the fees described in #3
5. Would the fees described in #3 be treated as foreign source income? Is there a rule on the ratio between foreign source income and Japan source income if the work performed domestically and overseas are similar? What if they aren't?
6. Does it make a difference if the payments described in #3 is paid into the individual's bank account or a different legal entity (wholly or partially owned by the individual) bank account?

Thank you very much and greatly appreciate any advice or recommendations on tax advisor.