At level 16 you will be given a companion quest. The reward is your choice of one of five companions for free. The Cleric Disciple, Sellsword, Man-at-Arms, Wayward Wizard and Dog are the reward choices and are tradable. All five are of white quality, max rank 15. All five can be purchased for 2gold each from the companion vendor who you will be directed to when you receive the companion quest.
Cleric Disciple - WHITE - Leader - 2g - Casts a heal for 20% of your max health every 30 seconds. Also has a ranged attack.
Angel of protection - PURPLE - Leader - 360 Ardent Coins - First spell heals for 5% of max health. Next two spells are wards that reduce incoming damage taken.
Acolyte of Kelemvore - BLUE - Leader - 2000 Zen - The acolyte will periodically cast a ward that will reduce incoming damage by 10% and a life steal buff. The lifesteal and damage reduction isn't much and won't be up often enough while in combat to offset the healing ability of the Cleric Disciple companion. Save your zen.
Acolyte of Kelemvore doesn't actually cast a heal spell. Instead this companion casts a lifesteal buff on the player that is enhanced by stacking lifesteal on the companion through equipment and enhancements causing the player to heal themselves through dps.