The relationship between Rufus and Tseng is one of my favorites in the entire franchise. You can look at it however you want, be it merely work related, perhaps in more terms of friendship or even more. However you see their relationship isn't that relevant for this.

I just want to talk a bit about these two, because their connection is pretty awesome deep down.

So, you got Rufus, this blond guy with white clothes, tricking everyone and supporting Avalanche to overthrow his father. He gets betrayed, but still finds someone who cares about his safety at the very least: the Turks. He owns them his life for that rescue. He even works with them in helping the Turks find and help Verdot and Elfe while Rufus himself is on house arrest, with the Turks no less, because who better to keep watch. He is also the one that keeps the Turks going, making it so that the Turks own him their lives. Years later, the Turks are by far the only ones that he trusts with his protection and important tasks, and even the ones that stay with him after the fall of Shinra. During this particular time, Rufus wears the nickname "ghost of Midgar" better than any other time, since he is staying low, hiding himself in Healen Lodge, away from civilization, but still trying to help the planet, step by step. I don't recall him actually ever being called "ghost of Midgar" in the series, but let's be honest: it super fits. I probably read it in some fanfic and it stayed, but it really looks like Rufus, his white outfits certainly help with that.

Then we have Tseng. While Rufus is basically a light palette, Tseng in turn is dark as night. Contrasting, and thus they resonate with one another. Tseng at first isn't really in charge of Rufus's protection, only starting to do so while Rufus is on house arrest with the Turks, as Tseng himself mainly stayed in the Turks HQ instead of the field. Tseng is also the one put on trial in charge of all remaining alive Turks, trial that is stopped by Rufus like said previously. Not that much relevant perhaps, but when Tseng lets down his hair, it's Rufus that is present. Rufus is the one that watches Tseng move from his torments, partly at least. As time goes, Tseng is basically Rufus's bodyguard among all the other Turks, and clearly the one Rufus trusts more and entrusts the more important tasks to. The dark colours that follow Tseng make him look like a shadow, and as he is frequently around Rufus, he would be Rufus's shadow to be more precise. When Tseng and Elena are attacked and tortured by Kadaj and his brothers, Rufus tries to negotiate, trading their return and safety for JENOVA, though Kadaj isn't very fond of those terms. Once again, Tseng is never called a shadow in the series, but the association certainly fits.

To top it all, both refuse to die multiple times. Just when you think they'll be dead, they appear alive. This both helps the "ghost of Midgar" title of Rufus as well as Tseng, who obviously has to follow, being his shadow and all that. Both also have their torments and debts and work on overcoming and pay them: Rufus is in debt to the planet, funding the WRO as a way to help, while staying low and hiding from civilization. Tseng isn't in debt to a whole planet, but instead to lots of people that marked his life, namely Verdot, Rufus himself, Zack and Aerith. Clearly these two won't die until their debts are paid, whenever that might be.

Lol, this is very messy, but I really like the relationship of these two and how they contrast and complement one another.