I think ZR id very carefully written so that you can borderline ship Sam with Runner 5 OR simply let Sam be like a brother to Five. Listening to most of the story beats, it could really go either way. They do it really strategically. Sam lost both his girlfriend AND his sibling early on in the apocalypse, so Five can replace either depending on the player's preference.
It is so interesting to go back and listen to A Voice in the Dark from s1.
Because I think you can argue that however Sam and Five are bonded, it happens in that Mission.
Sam is forever the character in the zombie apocalypse who still finds joy in the little things, and holds out hope even when everybody else gives up. But in A Voice in the Dark, when it looks like Five is gone, he has that whole super depressing voice clip where he says, "nobody stays sane through all this, Five," and goes on to muse about how "maybe next time I see you, I'll have to shoot you in the head."
And he's literally about to walk out and give up on Five making it back, and FIGURATIVELY about to turn a corner into being a less hopeful character. You get the sense that after seeing Alice zombified and killed recently, and after losing "another good runner," and spending the whole dark night alone thinking about how he lost his family, Sam Yao is about to drop the hopefulness and turn more cynical.
But then Five makes it back!
And he's back to being normal, ever-hopeful, ever-funny, sunshiny and caring radio operator. Whereas he COULD have turned out more like any other cynical survivor in the series.
So basically what I'm saying is, it's really well written, because not only can Five fill the shoes of the people closest to Sam whom he's lost, but Five can also represent the indestructible hope Sam always has. And in turn, while Five is out there running around facing impossible odds, there's always someone representing the everyman, the innocent and worthwhile, on the other end of the comms like an anchor.
In conclusion an exercise app has no right to be written as well as this one is 🤷♀️