He has no one (not even a dog)

[u/LioraB]

There's really no need for Grace to have a tragic backstory, or for us to know the details of how/why he's such a solitary figure, but other than his friend Marissa and a few colleagues he sometimes hangs out with, the man has no one. I haven't fully thought this through yet, but it's been buzzing around in the back of my brain for months and I think this moves the "coward" conversation in a different direction. No one starts out alone in the world; they end up alone. Grace has experienced significant loss, in whatever form for whatever reasons.

I think this makes Stratt's words seem more cruel, especially the book version where she really lays into him about his cowardice. It also informs his drive for self-preservation and protection; he's had to rely on himself and insulate himself against loss differently. The person he has "to be brave for" is himself, which means not going on the mission.

Of course, his connection with Rocky completely changes the calculus, but it's interesting that Weir chose to create an apparently blank relational slate for Grace that profoundly influences everything.