It was not illegal to help Chinese, but the climate in Japan in 1937-1945 was extremely bad for independent acts of bravery. The Public Security Preservation Law of 1925 outlawed socialist, anarchist and communist movements, and after this, there was a growing atmosphere that shunned Japanese who sympathized with the victims of their invasions or did any other sort of “anti-Japan” activity. For example, in 1937, the year of the Rape of Nanking, an outspoken anti-imperialist Christian named Yanaihara Tadao was forced to resign from Tokyo Imperial University simply for voicing opposition to the war from the comfort of Tokyo. (In 1945 he was hired back, and in 1951 he became the university's president.)

The closest case we have to Japanese assisting the Chinese on the ground during the Rape of Nanking that I know of is a group of newspaper reporters who yelled to a Japanese Army unit that the people they were shooting were civilians and not soldiers. This group includes Imai Seigō 今井正剛 and Nakamura Shōgo 中村正吾. Reportedly they saved the lives of two men who they recognized as employees of a tailor shop they frequented. Their story was published in 1989 (citation: 『目撃者が語る日中戦争』P53-P55).

Besides these two there were other reporters who saw what was happening, including Moriyama Yoshio 守山義雄 and Ōya Sōichi 大宅壮一. They were not able to tell the details in the Japanese press due to censorship, but they told an attaché in Shanghai named Itō Nobufumi 伊藤述史, who secretly contacted his superior, Foreign Minister Hirota Kōki, to alert him to the atrocities. This was not made public, and Itō only acknowledged it reluctantly at the Tokyo Trials.

At this vaguer level, Prince Mikasa-no-miya was dispatched to China in 1937 and also alerted his superior, namely, his brother Emperor Hirohito. He showed Hirohito a film of Japanese atrocities, pleading with him to stop the invasion. It seems that the Emperor was unmoved. It has also been reported that Mikasa-no-miya requested and was shown a film of Unit 731 activities. He is the only Japanese military officer I am aware of who was inquisitive into these matters.

We have no testimony of any Japanese inside Unit 731 trying to save Chinese from human experimentation.