Summed up in one sentence: Annoyingly hard to catch. He’s a ranged ADC with very low range, but a whole host of mobility tools. He has no ultimate, and instead has four basic abilities, similar to Udyr. I think it fits his theme better.
His passive works similarly to Kha’Zix’s ultimate: While it is off cooldown, he can improve one of his abilities, and the passive will go on cooldown. The cooldown is rather long, so he has to think carefully about which improvement to have at any given time.
His Q fixes his range problem temporarily, but only against a single target. It goes on cooldown only after it is disabled, so he has to choose his target carefully. The target can drop his range by leaving vision, but otherwise it lasts until it is disabled.
His W is sort of like a dash, but it’s a movement speed boost instead. It adds 600 units in total distance travelled over 1.5 seconds.
His E is basically link’s hookshot from the zelda games. You fire it, and you can dash to it like a thresh lantern. It’s a decent escape with a lot of interesting options, but enemies can intercept it, so it’s not a totally reliable escape. It has a long cooldown at all ranks, but augmenting it gives you two.
His R is sort of like twitch’s e in that it does more damage if you attack more. The difference is that it gives you a window to attack and then deals damage after that. Augmenting it increases the window if you’re attacking constantly.
Alltogether, I can see two build paths for him: Either he could go glass cannon ADC, mainly relying on good target selection with Seeker’s Darts to deal damage safely, and securing himself with rockets or hookshots when he does need to attack things without the safety of Seeker’s Darts.
Alternately, he could opt for a more bruiser-ish build, exploiting the fact that while his autoattack range is low, it’s higher than melee champions. Frozen Mallet and Trinity Force would allow him to kite reasonably well, and he would have decent burst from his R because he would generally be able to get more autoattacks in because he would have more time with them in his range (assuming seeker’s darts is used in conjunction with Blasting Charge)
Lore wise, i’d expect him to be good friends with Ezreal.