With a duo, if the second player is walking around the ship/not aboard, Buddy will man the guns. If the second player is manning the guns, it will reload the guns (even crafting the required supplies when doing so). It does not seem like it will put out fires, or do repairs.

In solo, he will use the sensors, he will man the guns, perform repairs, empty the materia collectors, accompany you on space walks, carry items back to the ship for you… Basically a ton of stuff.

What I was really impressed with is that Buddy still helps out in a duo. We would have been in trouble otherwise. Keeping the ship together during a big fight is a full-time task, so when a fire starts up, mines grab onto the hull, or repairs become necessary, the second player can hop off the guns and Buddy will take over while you put out fires, fight off boarders, etc. It’s great.

And reloading the guns just means a duo can keep fighting as long as nothing is going wrong with the ship.

Essentially, you get to do the fun and chaotic parts, while Buddy picks up any slack when too many things need doing at the same time.