I'm kinda concerned that they said that adding Karlach to our existing companions 'rounds out the spectrum' of morality, from good to bad and grey in between... because right now we have origin companions that cover: CE, LE, NE (possibly un-brainwashable to NG), TN and CN; even if Karlach is CG that's not 'rounding out a spectrum'.
Actually, they wrote this:
She will join Astarion, Gale, Shadowheart, Lae’zel, and Wyll like a well-meaning, devil-fighting bat out of hell, rounding out the moral spectrum of playable characters from good to bad, and the shades of grey between.
This is not super clear, but imo this statement leans more towards "we have good, neutral and bad
playable characters in the game".
Playable characters is the key word, players are given the diversity of moral values from good to evil for a potential origin playthrough. Watch HarbsNarbs' videos on companions, it is clear that every origin will have breaking points where you can shift your character's values by following different paths. There was also no proper setting in EA where the Law/Chaos system could be properly explored for each of the party members. I still feel you and wish for the full roster as well, but my feeling is that what Larian meant to say might be a little different from your interpretation.
