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.
Zombie Ward's additional vision scales incredibly well on Lee Sin thanks to his natural high ward placement with Tracker's Knife. The extra bit of vision will scale well into the mid game for securing objectives like Dragon and Baron.
The majority of Lee sin's high damage comes in the form of his abilities, specifically [Q] and [R]/ Ravenous Hunter allows you to skirmish often thanks to the extra bit of healing.
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.