yes, its talking about one your NPC friends. you can equip them with any gear you find, just click on their portrait in the upper left corner to open their inventory if your paired up with them. otherwise i think you can equip them from where they are standing in town, just click on them.
For the Demon Hunter its the Shadow Mantle set. Each character gets one set for completing the seasonal chapters, so for the Necromancer you would receive the Grace of Inarius set. You don't get anything for completing chapter one for some reason, but chapter 2 gives you two pieces, chapter 3 gives 2 more and chapter 4 gives the final 2.
you can check it out here:
https://www.icy-veins.com/forums/topic/ ... s-rewards/
looks like with that set these are the bonuses you get for having either 2, 4 or all 6 pieces of gear. Looks like bone armor gets a huge damage increase. Never played the Necro though, i don't own him.
Bone Armor damage is increased by 1000%.
Bone Armor grants an additional 2% damage reduction per enemy hit.
Bone Armor also activates a swirling tornado of bone, damaging nearby enemies for 1000% weapon damage and increasing the damage they take from the Necromancer by 10,000%.