First, I love his lore and the sniper support idea. Second, I believe the word you're looking for is scaling, not adaptive. Adaptive means that something interacts with ad or so or does physical or magical damage based on whether you have more ad or ap. Also, you might want to indicate whether it scales with a specific stat or with ability rank. onto his kit:
His passive making his damage scale with kills from outside vision range and his q range scaling with vision score seem like poor ideas. The former is an unnecessary snowballing mechanic with awkward conditions and the latter feels unintuitive, makes him want to spam wards regardless of them serving a purpose, and doesn't work for certain maps and game modes.
My suggestion would be scaling but non-stacking bonus damage related to striking a target spotted by one of his wards or abilities to get rid of the reliance on kills, reward smart warding, and give it a way to work without wards by spotting with other abilities. The kill sharing portion is solid.
The second part of the passive is mostly fine though you might want to also give him the reward for kills also or my personal favorite which would make it work like dark harvest where damaging or impairing a low health target gives the reward with a cooldown per target so he can more reliably get his upgrades. If you go the second route then you could also require being out of the target's sight range to stick with his theme.
His e is really cool in concept and to let him stay out of sight for his passive. The only issue is that latching to enemies feels kind of wasted since he never wants to be that close to an enemy. My suggestion would be to make the strike performed when detaching deal additional damage against low health targets and roll away from the target so he can use it to either finish off a low health target or get a bit of distance in a pinch. Also should probably have a much lower duration for enemies than for allies. Can he use abilities while attached? If not then I'd suggest it.
The w is pretty interesting concept but I have a few issues with it depending on how long it would take for the portals to open. If they open quickly then allies would have issues with getting teleported when they don't want to be. If they open slowly then it wouldn't be worth it for allies to take them in most cases. Also, if the range on this is long enough then I really like what this would let him do to support allies from a distance and either use the slow from q to set this up or use the slow from this to set q up.
Here's my idea for a set of revisions: portals open fairly quickly (Taliyah w might be a good comparison) but only teleport enemies upon opening to make it less stupid in chokepoints and stop them from chasing allies through the portals if they aren't already right on top of them. That would also let you give them a longer duration so he can set up an escape route in advance. Either portal gives vision and enemies teleported are revealed for synergy with the proposed passive. Allies are no longer teleported automatically unless they're stunned when it opens. Allies can right click the portal to teleport similar to Thresh lantern with at most one teleport per cast per person. Movement speed is granted upon touching the portal rather than teleporting so it still helps even if your ally refuses to click the damn portal.
You could potentially make w vector cast to avoid situations where the second portal isn't placed in time but I would advise against it because patch 6.14 was stupid it would make the ability go off more slowly and decrease the options at his disposal. If he doesn't place the second portal in time either cancel the ability with a partial cooldown refund, make a single portal show up and just apply the vision, slow, and speedup without teleporting anyone, or just wait for him to place a second portal for the spell to go off.
His ultimate kind of feel a like a knockoff Nocturne ult but with his kit and a lower cooldown it might be kind of interesting. I'd suggest also revealing enemy champions in the area for an added bonus and synergy with the proposed passive. If you take the suggestion for his q to have multiple charges then the number of charges could scale with ult rank to give him better power spikes.
The Q is really solid, I'm assuming it works similarly to Jhin's w. Two things that might be interesting would be multiple charges so he can use it more in a fight and is encouraged to hold onto it for the right situation rather than spamming it whenever It's off cooldown, and making it root, ground, or apply nearsightedness when hitting a slowed target to reward hitting his other skills and let apply a bit more utility to help his team out. The slow could also be increased while he isn't spotted.
My suggestion for the charges and ult would be 1 charge base, 2 with one point in ult, and 3 with a second point in ult with ult only having two ranks and q having six similar to Ryze. Lockout between shots should be longer than the slow.
The upgrades he can buy feel really lacking. The first one helps him a lot when he's getting hit when the rest of his kit is about staying far away from enemies and the second is just a flat damage boost. Here's a few ideas for upgrades, each of these would be a single upgrade you would buy that would let him modify his kit based on the situation:
Crippler Rounds: this could either give the additional crowd control for landing q on a slowed target, change the additional crowd control provided (my favorite), or slightly increase the duration of the empowered crowd control.
Double Tap: Reactivating q during the first half of the lockout between shots with ammo available fires a second shot that doesn't apply cc, ignores non-champions, and has reduced base damage but deals bonus execute damage.
Lingering Shadows: portals grant vision enemies for longer and apply the reveal to enemies who pass by them, not just those who are teleported.
Contingency Planning: Teleporting with w gives a shield scaling with missing health and (only works for Scaith). Portals linger until the ability is used again.
Rapid Reload: Detaching from a champion immediately reloads up to two charges of q.
Shadow Burst: Detaching from a champion releases a burst that slows nearby enemies.
Extended Barrel: Increases range of basic abilities. Wards can be placed from further away.
Living Shadows: Q leaves behind a living shadow that casts the ability for him.