The Apprentice is a Name that studies the mysteries of magic. The name is often seen in Praes, as a student of the Warlock. Both known Praesi holders of the Name have shown an extreme disinterest in matters unrelated to magic. It does not appear to imply a magical specialty, such as the Conjurer, only that the holder is a mage of uncommon power.
In Book 6, a new heroic Apprentice from Ashur appears, proving the Name isn't tied only to Praes, nor by default belonging to Villains.
Like Squire, Apprentice is a transitional "student" Name, for individuals clearly meant for magical greatness but not yet settled on a specific Role. It lasts only a few years at most before the holder finds their formative pivot and transitions again. Former Apprentices can become any of a wide variety of mage Names, such as Wizard, Warlock or Heirophant.
The holder has increased magical capacity allowing them to perform spells with greater power, as well as weave spells with greater accuracy. Notable accomplishments of previous holders are: have performed magical soul surgery, restrained Fae royalty and demons, and created pocket dimensions.
Sapan: Originating from Ashur, Sapan was originally studying wanting to be the Silver Mage, but abandoned the healing spells when her mentor got killed during the sack of Smyrna. After signing onto the Truce and Terms she has picked up a number of fast and easy war magics. While simple, Sapan's spells are also swift and highly destructive, and Catherine Foundling believes it'd take a Revenant to trouble her. 
She is described as "a girl in dusty mages robes with a bony face". 
Masego: Masego was adopted and raised by Wekesa and Tikoloshe. Until the age of nine he was kept in a demi-plane between Creation and Arcadia, at which point Wekesa decided he was old enough to return to Ater and allowed the construct to collapse. Watching his world crumble before his eyes impressed upon Masego the artificiality of Creation and left him with a desire to go beyond it. He became the Apprentice and came into his first Aspect complaining about another mage's work, which likely drove his major flaw of insistently correcting people. Masego eventually transitioned to the Heirophant.
Wekesa: Wekesa gained the role of Apprentice while studying under the then Warlock. He later fled his master and joined with the revolutionaries. Eventually he returned for revenge against his former master. He exhausted all his magic before the battle began in order to launch open up a portal causing hell ire to drop down on his master. Killing his former master allowed Wekesa to transition to becoming the Warlock.