Warning: spoiler about the ending with a specific companion
spoiler
Lae’zel
I mean, I did everything she said, I saved her comet crush, I saved the world, we had the top of the relationships, I was “the source of her joy”, and then, just because I had to transform myself into a flying squid “she can’t cross that line” !1One!! :D
In my headcanon, after the big brain, I did track down and exterminate all the Githyanki after she left >:|
It’s her epilogue and not and ending tho. It’s the ending of the relationship questline/development.
Every single companion is going to be involved in the endings because they’re part of your party. Not all of them will be there, but they all are involved by just being at your camp.