I like this idea but i think it needs more tweaks.
I'll try to review a little bit.
-You say all skills except assassination have 600 range, there is no skill called assassination that I see.
-His Passive, I love this idea, but there are some concerns i have and will expand on in a minute.
-His Q, seems a bit too op, maybe make the AD scaling 75% instead of a full 100% if so then the base damage is fine where it is. I think the 150% damage increase should be if the target is stunned at all, not just if they are stunned by Reaper 09.
-His W, I'm having a hard time understanding this ability, it says he is stealthed for 1.5 seconds, but he can't move. now i guess that's fine but if he becomes a champion then people will know he cant move while stealthed and 1.5 seconds is too short, people will see you and where you go invisible. so the second you do they will throw an AoE at your last know position and you can't do anything about it.
my recommendation for this, either 1) make your stealth longer and keep the immobility. 2) make Reaper 09 able to move around during stealth (so he can line up his point blank shot, or even escape maybe.)
also for his W, you have a movement speed boost for in and out of combat, do they stack if you are in combat? just wondering.
His E, I think this idea is pretty solid, you could add more time to the stun, like make it a 2 second stun, or if you leave it at 1 second then add a slow that lingers after the initial stun wears off. so they get shot on the knees and when they recoup they are basically crawling away for a few seconds.
His Ult, I like this concept but i think they way it is laid out or at least the way i see it, it is immensely OP. basically it says that at level 16 he will do at least 1200 base damage with his ult which is kind of ridiculous and if you are carrying your team there could be under-leveled people you may be able to instantly kill with this.
Also remember how i said there was a problem i had with your passive, here it is. according to your ult, it doesn't say if 6 bullets is the max that can be fired, so if you timed your activation correctly you could fire 7 shots, maybe even 8 depending on the circumstances and cast time involved.
my fix idea for the ult, I think you should back way off on the Base damage output and make it scale like 50% off AD, that way it isn't so OP right from stock. make it like 25/50/75
That way, say you have like 300AD late game, and it scales off 50% of your AD, then it would do 75(+150) x 6 = 1350 damage. that seems much more reasonable
and to eliminate the whole "timing it right and getting more than 6 shots off" thing i spoke of, you could simply make it a 6 shot cap, or make it fire all the shot immediately for a big burst shot and get rid of the select another enemy within 1 second thing.
Finally, your Lore. I like the story but it's so short, you should expand on it, like give the secret organization a name, and tell us how he narrowly escaped death with them. that way it will lure a reader into it much more than giving a brief background check on him.
Just my thoughts, good concept.
If you have time, can you check out mine, I'd love feedback.