I don't generally do much with Harper though I know they use hypnosis when treating the player. I think you can get hypnotized while in the Asylum, though I only went there once so I may remember that wrong.
The problem with Harper is that it's a scripted event, so even in DoLP combat with Harper isn't going to last all that long. Harper
can use hypnosis but from my experience cycling in and out of the Asylum you're going to be incredibly lucky (or unlucky depending on your POV) if Harper manages to hypnotize the player twice, and somewhat lucky if it even happens once.
(Edit) And yes, changing your actions does affect results, so you could save-scum a Harper encounter to hopefully work on getting hypnotized, but honestly that level of dedication is not worth it.