Baldur’s Gate 3 is a story-rich, party-based RPG set in the universe of Dungeons & Dragons, where your choices shape a tale of fellowship and betrayal, survival and sacrifice, and the lure of absolute power. (Website)
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There is no hard coded consequence. "Attitude" is single character.
That said if he is in your party there is a chance he will be caught for some Dialogs and this will count against him by region.
There are more than a few checks that if he is the last damage dealer in an encounter he will initiate dialog. Regardless of if he is the MC.
Do you mean attitude towards characters? It only happens to Astarion if he's the one caught stealing. Though if it gets him in a fight, your party fights too if they aren't separated.
Yes, attitude. Thanks.
There's no consequences if he doesn't get caught. Separate him from the group, sneak around, steal, and then return to the group. The only drawback is that if he gets caught and ends up in a fight, it's going to take you awhile to help him. He can just dash away though if you give him any movement boosts. But be forewarned. There are serious consequences to getting caught in some parts of the game, very serious consequences.