Gale's delivery is manipulative. It's in the structure of his 'asks'. He butters you up then he ask for your consent in a manipulative manner. He tells you he trusts you but refuses to divulge more info.
Interesting. I've certainly read the bum notes in the way he goes about trying to engage our cooperation as social awkwardness and anxiety about our reaction rather than manipulation, but I can see how others might take it differently. And given I've said I didn't trust Shadowheart partly because of her flattery, cosying up and conspiratorial manner you've got me thinking about why I'm not getting the same vibes from Gale. Part of it is surely that he's not (as far as we know

) a Sharran, and unlike Shadowheart I don't think he actively tries to stir up distrust against other party members. But I guess it's largely because, once he does open up, I believe him and don't see any obvious point of conflict in what he wants and what I, or my characters that would have him in his party, are likely to want to achieve.
I guess it makes sense that we can read these same characters in very different ways, as presumably they have been designed precisely to be malleable so that they can be developed in different ways as both origin characters and as party members depending on our choices. It wouldn't surprise me to find in the full game that in some playthroughs Gale can be a basically good if somewhat over-ambitious man who made some bad choices for love, and in others can actually be an unscrupulous seeker after power who was trying to equal his goddess rather than win back her affection. I suppose that his character might be more fixed as a companion than a protagonist, but even as the former there's plenty of room for the writers to tweak both his history and choices in the game in response to the player. There's probably no sensible answer to the question of who he, or any of the other companions, really are as that's going to change from game to game.
you get the point to gale's personality!
gale's original intention should be trying to equal mystra.
but i am afraid that gale's final goal will be qualitative change -- "devour mystra, just follow the steps of karsus."
the nature of the "dark magic shard" is devour "magic shard".
if gale doen't choose return the dark magic shard to astral world and try to master it, gale will follow the steps of karsus.
though gale shows the way to return the shard to astral world, but i think this is his secondary consideration.
gale's first consideration is still mastering the power of the shard.
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by the way
i suggest the "origin characters" shadowheart, lae'zel, gale, wyll, astarion, they are just companions, they have their legends, but they are not savior of toril, becuase they have already had their strong personalities.
the savior of toril, only our player character who is the custom character that is created be our players own.