What does the Companion link do in co-op?

The butterflies do double duty: they show the distance between you, and they tell you Companion Mode is on. If the line of butterflies is there, you are linked. If it is gone, you are not, and your partner can wander off without you noticing.

The point is keeping the pair together in an open world. When one of you peels off toward a creature or a landmark, the tether stretches across the gap. You can see which way to go and how far without stopping to ask. The link shows you where your partner is. It does not keep them close for you, so the job of staying together is still yours.

Co-op takes place in the host's main world, which works as a private instance. Invited friends can explore and catch creatures there together, and that is where the link earns its place. Two people catching in the same area drift apart quickly, and the tether makes it easy to regroup before heading on.

Where staying linked matters

Some Sanctum puzzle dungeons need a party of at least two players, so you have to arrive with your partner. World bosses and Alpha Aniimo bosses can also be fought cooperatively, and getting both players to the fight together is easier when the tether shows you how far apart you are.