they are not friends. they went through the similar struggles and similar losses, they became close because of it. it's not romantic or friendship. more like a family. they are basically coworkers, jing yuan hired yukong and watched her lose her best friend and quit flying. yukong watched this old man suddenly take in an abandoned child and become a little bit more lively again. full of love. they both are raising kids and being not so good at it. yukong can't help but see qingni in yanqing. she misses the time her daughter was little. and she can't help but get a little angry at him for doing the opposite of she did. allowing yanqing become a soldier. but the thing they are similar is that they both are hiding away something from their kids. i think their relationship is not so smooth as may seem. they know eachother very well and hold each other dear but also they often fight over many things, starting from job matters and questions about raising children ("ahh don't tell me how do i raise my child" – "oh no i will because you asked me how do you do it and now you are doing it wrong and everyone but me is afraid to say that to you") and finishing with the strategic questions and philosophy, their varying point of view about life.