in honor of ppl calling astarion chaotic evil for some reason, lets look at the players handbook alignment section, specifically the three chaotic alignments: "Chaotic good (CG) creatures act as their conscience directs, with little regard for what others expect. Copper dragons, many elves, and unicorns are chaotic good." "Chaotic neutral (CN) creatures follow their whims, holding their personal freedom above all else. Many barbarians and rogues, and some bards, are chaotic neutral." "Chaotic evil (CE) creatures act with arbitrary violence, spurred by their greed, hatred, or bloodlust. Demons, red dragons, and orcs are chaotic evil."
astarion does not act on arbitrary violence, and even admonishes you for doing so if you play durge (the urge during act 2), because you could have had better chances and influence if they were alive. he cares about his own freedom, and power is a method to get and keep it. he does not care about violence either way when it does not suit his bids for freedom through gaining power. he is chaotic neutral, that's why he will stay in both a 'good' and 'evil' run of bg3. the only evil companion is minthara. companions are not accessible if you play in a moral alignment contrary to their own (i.e. wyll and karlach leaving during an 'evil' run and minthara being inaccessible for a 'good' run). so astarion staying during both alignments means he is neutral.