Yi is considerate one of the hardest carries in the game, As an On-hit oriented Blademaster, Guinsoo's Rageblade is considerate his Core item, after that is recommended to build him tanky with items such a Dragon's Claw and Warmog's Armor to keep him alive for as long as possible. Rapid Firecannon is also a good option due to his fragile melee nature.
Alternative builds include turning him into a Berserker, granting him the ability to cleave through teams with ease.
Out of all Mystic, Nami complements him the most, as she can empower his already strong On-hit auto attacks and set-up initionations for him with her Tidal Wave.
Active - Meditate: Meditates, becoming untargetable for 1 second and healing 30% of maximum health over the duration. Afterwards, for 6 seconds, gains 100% bonus attack speed and basic attacks deal 50 / 75 / 500 bonus magic damage on-hit, but cannot gain mana for the duration. Meditate ends if he is full health.