Astarion is an Origin companion whose default class is Rogue. That makes him a natural fit for Dexterity-based combat, stealth, lockpicking and other skill-heavy exploration tasks.
Best role for Astarion
In a standard party, Astarion is most useful when you want one character to cover both exploration utility and reliable single-target damage. He is especially convenient if no one else in the group is built around Dexterity or Sleight of Hand.
When Astarion fits your party
- You need a dedicated lockpicker or trap disarmer.
- You want stealth and scouting without giving up combat damage.
- Your frontline and support roles are already covered.
- You want a companion who can later branch into a Rogue multiclass.
Build direction
If you keep him close to his default identity, prioritize a clear damage role first, then make sure the party still has enough frontline pressure and support. Astarion works well in parties where another character can keep enemies occupied while he attacks from advantageous positions.
Common mistake
Do not choose party members only by default class. BG3 lets you respec companions, so the more useful question is whether you want Astarion for his story and personality, then what combat role your current party still needs.
Source: BG3 Wiki — Origins.