Non-Player Characters or NPCs are characters found in the games that cannot be controlled by the player.
Shell comes and goes from the Hideout and sells you weapons, gems, and supplies at a discount, or sell your mastered items. She can come more often, give a deeper discount, or sell more things using skills. She is "recruited" after Siris's Act I.
Jensen is recruited in Act III. Jensen is a childhood friend of Siris and a skilled Blacksmith. He upgrades your items to either upgrade stats, add slots and transform normal gem slots into star gem slots.
The Gemcutter, Burgess Benedict Batteric III, forges gems for you at the Hideout. He is recruited after Isa's Act IV.
Eves is the Potionmaster, and one of Raidriar's Devoted before the God King was killed by Galath. He can create all sorts of different potions using the ingredients you buy with Chips from the shop, or collect in the game. Any potion can be made, from an stat boost to a rare health potion to a elemental attack addition. He is recruited at the start of the game.