I don't think it's the queen, though I did at one point- now I think it's someone of the queen's bloodline, someone older than the queen even.They aren't Biologically immortal, Each species/type of giantess has different lifespans... And Possibly?... Maybe He remembers that smell from first meeting her?? Then again, the MC (We) don't mention it... The queen I say is one Higher possibilities but a part of me is saying it's someone that on par with the queen or even higher.. (Void Goddess?)
I wouldn't think if it was just the smell from when he first met her that he wouldn't be able to place it, which makes me think it's a scent he's able to recall from before he woke up in
And, fair I suppose, I just assumed giantesses were biologically immortal due to how obscenely long-lived they all are, with the queen having lived for over 3500 years (that's only how long she's been queen, there's no telling how many centuries or even millennia she lived beforehand) and still lookin' good, and not diminished in strength at all (at least as far as we know).
We don't have an age for Regina, but she's probably got a millennia or two under her belt, and then there's no telling how long that means her mentor has lived.