Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.
Lee Sin can very easily proc Electrocute as his [Q] counts as 2 unique spells. A quick auto after [Q] and Electrocute will go off immediately for that extra bit of burst damage and kill secure.
Dashing with either [Q] or [W] makes for an easy way to get a quick 10 Lethality, giving Lee plenty of early game kill potential.
Eyeball collection on Lee Sin allows you to snowball your strong early game presence if you're able to successfully gank.
Lee Sin is all about his positioning and getting into the lane for a gank as quickly as he can. The extra Movement Speed from Relentless Hunter plays a big role in keeping you on the map and relevant everywhere.
Triumph's burst of bonus healing after a kill/assist allows you to get away with some risk tower-dives and survive so that you're able to snowball a lead to victory.
WhyCoup De Grace
Coup De Grace synergizes well with Lee Sin's high damage and execution nature of [Q]. Combined with Electrocute and Ravenous Hunter, Lee is an incredibly strong dueling jungler.