Guy best friend is your non-romantic male soul mate. Usually when a girl is deprived of a brother, this male ends up being the replacement. They are as close as siblings and he means the world to her. He is referred to as the "guy best friend" usually when the girl has best friends who are mostly girls. 

Guy best friend（男闺蜜）就是跟你没有恋爱关系的男性灵魂伴侣。如果一个女孩失去了兄长，这位男士就会成为替代品。他们亲如兄妹，他是她的全部。因为这个女孩的好朋友大多都是女性，所以，这位男性好友就被称为“男闺蜜”。

While most of the time these two remain best friends, it is usually the male that tends to fall for the female. Females who do fall for their guy best friend usually keep it quiet in fear of losing their best friend.

大多数情况下，两人都会一直保持好朋友关系，不过，男方通常会对女方产生情愫。如果女方发现自己对男闺蜜产生感情，一般不会表白，因为怕会失去这个好朋友。