10M jpy Money transfer

Need advice on Japan banking/tax implications.
Someone close to family wants to transfer 10M to my bank account in Japan.

Context:
5M is a genuine loan to me, and I plan to pay it back.
The other 5M is NOT mine. He wants me to temporarily hold it because he and my mom have spending issues, and he trusts me to keep it safe.
He lives in Japan and is Japanese.

The money would be bank-to-bank transfer.

Questions:
Would there be tax implications for me?
Would my bank question a ¥10M transfer?
Is holding someone else’s money in my personal account a bad idea legally/banking-wise?
Is there a cleaner structure we should consider?
Trying to do this properly and avoid problems later.

by theycallmeCinderella