[@KatherinWinter]
To quote Will Smith:

Nah nah nah.
The good guys dress in black, remember that
Just in case we ever face to face and make contact
The title held by me, MIB
Means what you think you saw, you did not see
So don't blink be what was there is now gone
The black suits with the black Ray Bans on
(skip a few verses)
With the quickness, talk with the witnesses
Hypnotizer, neuralizer
Vivid memories turn to fantasies
----
Basically, witness containment specialist. Like a living neuralyzer but more effective, more precise, and better control over what falsified memories they are given to cover up the missing time. Also no risk of brain damage if the same unlucky bastard ends up having to be dealt with multiple times. Trim off the last, say, hour worth of memories. Set them up with memories of a blind date that didn't go so well, and you're in the clear. Or a blind date that did go well if you've got enough people to work with.

It'd be slower than just neuralyzing them, but the end result would be a lot cleaner.

They've also got the ability to heal rapidly (in the original Islamic mythology, Djinn are made from 'smokeless flame'), so I basically made it so they can repair their bodies by consuming combustible materials to replenish their internal flame; plus the ability to switch between their Djinn and Human forms but that doesn't impact their abilities besides the human shell not being able to use their memory magic. If you REALLY want to prevent him from altering memories when not on the job, have him wear something that locks him into his human form when he isn't in the field dealing with witnesses.

I didn't figure the rapid healing would be an issue (or even a big deal) since that's a common feature of a lot of fantasy races.

EDIT: After I typed this up, I had a thought. If you allow me to use this version of Djinn, it'd provide a good way of decommissioning characters in-RP if their player decides to leave without having to just pretend they didn't exist. Just have him spend a couple hours with them, talk to them, formulate a good cover story for why they've been away from their old life for so long, then trim out the real memories and replace them with hollows; none of that 30 year coma nonsense they did with Agent K in MIB II