At first when I read the idea I thought of Marcone, he is getting to know "the Unseelie Acccords forwards and backwards", yet he isn't one of the magical beings.
However he is significantly a different archetype from the pseudo sidekick type you mention. However there are certain semi-political parties already in play in fantasy like Dresden Files. The Merlin is basically a judge, the rest of the council is like a jury, and usually the defendant's represent themselves unless another wizard steps in to assist.
Basically there isn't much use for a lawyer character unless the lawyer has significant ability to actually protect your rights, however that is kind of already covered by Lea in DF.
In addition Peabody sort of played the lawyer type, I don't think it's uncommon for a lawyer to be involved in the finances and treasury of an organization.
However these are all still not the character type you describe, I think this lawyer character sounds like he would be somewhere on the same level as Vince the
private eye that Harry hires
.
Then there is also BOB the Skull, I feel that he also provides much "legal advice" for Harry.
So in Dresden between Bob, McCoy, Rashid, Lea, Marcone, etc. I think Dresden has all his bases pretty much covered.
But I definitely think in a different story that your character would be very neccessary.
Kind of a Sherlock Holmes and Dr Watson set-up it sounds like would work.