The fact that he can attempt to mutiny the party puts him in the category, to me, but he's definitely not aggressive in the same way that Cassandra is. She's a hot head that just starts screaming and threatening people right off. Sten was a lot more together, he seemed more disciplined and definitely more in control of his emotions. I guess it just comes down to me being uncomfortable with characters that want the lead, on a personal level. I favor the "laid back rogue" types that crack jokes and come along for the ride. Like Varric, Isabela, Zevran, Awakening Anders.
Don't get me wrong, though, it doesn't make them bad characters. There's line between "I like you" and "I like what you add to the story". I absolutely didn't like Sten as an individual. I don't like his belief system and I don't like him. I do like what he adds to the story and I think the Qunari are very interesting from an outside perspective. So far, nobody in a DA game has failed to meet the "I like what you add to the story" category, for me.
I just can't blame Sten for going "WTF!?" at that point in the game. My own (as the player) reaction was pretty much "I completely agree with you, searching for the alleged burned ashes of some religious prophet while all of Ferelden is burning is insane and stupid, but the plot won't let me do anything else!".
I don't mind people who want to lead - they're just people that I, as the actual leader, have to knock back down and keep in line. Personally, I prefer people who are openly combative like that - the alternative is rumours and politicking, and that I can't stand.
And I totally get the distinction between "don't like as a person" but "like as a character". I was just musing that I didn't find Sten to be aggressive, I suppose to me someone being aggressive has to involve them trying to do actual violence to me in a way that phazes me - which I get is a totally useless idiosyncratic definition, but I think that's partly because I'm told I can be very aggressive IRL.
To bring this back on topic, I kind of wish for an aggressive LI, so if that's who Cas is, I think it'll be fun. It's just nice to have someone who stands up to you, not because of some contrived plot reason, but because that's who they are as a person. I like people who push back.