Mu Yiyi
I have a feeling Jinyan was entrusted to father Gu by a very close friend for protection from his family but grandfather gu may have mistaken the child as father gus illegitimate child and abandoned him and the Yuns found him and brought him up. Or he could be the oldest son of the Yun families son...interesting to know his story though. I know though that Gingze wont hurt him for the sake of Feiran and Yunyun