Ren Dan beard refers to the eight-character beard on the trademark image of Ren Dan, Japan, which is very popular in Japan and not uncommon in the upper Soviet Union. At least half a dozen generals have this beard.
It originated in Heian period, when officials in four places were closely related to Mao. He was at odds with the then Taisho minister Fujiwara. Fujiwara's "servants" all have moustaches, and the big partner ordered his own "servants" to have square beards to show that they would never compromise with Fujiwara.
Later, Fujiwara's family designed to encircle his big partner Jin Mao in the name of imperial edict, and said that followers of Jin Mao could leave safely as long as they laid down their weapons, and if they could capture or kill Jin Mao alive, they would get a reward. None of the more than 70 "servants" of Jin Mao surrendered or defected, and all died under the leadership of Jin Mao. This spirit was rewarded by everyone, including Fujiwara, and Chongan personally set up a monument for them.
Since then, the square beard has become a "symbol" of many warriors, expressing their loyalty and fortitude. That's why they grow beards. Generally speaking, in Japan, if a warrior with a good family background and high status or a descendant of a warrior (for example, a person with a family tree of 500 to 600 years) has a moustache, then the square beard is left by a person with a lower family background (for example, a veteran of Japan before World War II, who often had a square beard, was a businessman from his ancestors and a civilian from his grandfather).