Sure if you only play through the other house's routes you could come under the impression that he's just a slippery asshole who goes to whatever lengths possible to enact his liege's will(a role he's still great at), but when you really get to know Hubert by going through the Black Eagles route, particularly the Crimson Flower path, you can see how complex a person he really is.
Hubert at first seems like he follows Edelgard out of duty for his station then when you find out about the guilt he felt when he wasn't able to prevent her kidnapping to the Kingdom of Farhegus you might be inclined to think that that is the reason that he's so loyal to her. The truth is though that Hubert earnestly believes in Edelgard's vision for the future and wishes to be a part of making that a reality.
There's even a twist on the idea of the 'dark mage advisor' going behind their liege's back to do things that the liege wouldn't approve of. In most cases, this character does it for the reason that their liege is too soft or lacks the will to do what 'needs to be done', and Hubert does go behind Edelgard's back to do things that she wouldn't approve of. The subversion is that the reason Hubert goes behind Edelgard's back to do unsavory things that she wouldn't approve of is that he does not wish to add even further to the burden of guilt that Edelgard experiences for the things that she feels that she must do to make a better future for Fodlan.
Even his development with the other characters is fairly unique. You might expect that him developing stronger relationships with the other characters would result in him becoming less devoted to Edelgard, but his social development is actually him becoming nearly as devoted to his comrades as he is to Edelgard herself.
I could go further, but I don't want to turn this into a whole essay lol, anyway stan Hubert von Vestra, the best 'dark mage advisor'!
Also as an addition what other characters of Hubert's trope do you really enjoy?
