First of all: Yi already has the highest base movementspeed in the game. improving it with relentless hunter gives u the ability to quickly move towards different lanes and events happening on summoners rift. ghost is primarily for catching people and being able to dodge incoming skillshots with ur high MS. gustwalker helps u to clear ur jungle sides EXTREMLY fast. its also the best option in my eyes, since the other jungle pets are not ideal for yi now. we do not have to talk about scorchclaw (its ****) and moostomper is ok-ish, but since the tencacity got completely removed, it just doesnt fell strong anymore just to have this little shield.