Each of your four companions has its own set of abilities that you can choose to unlock and upgrade from the "companions" ability tree. These trees are much smaller than the player's, and contain between 4 and 5 abilities for each companion.
Progression[]
As a companion levels up, they acquire points which may be spent on unlocking one of their abilities. Points may also be spent on upgrading a companion's learned abilities, instead of learning new ones. However, unlike player abilities where all upgrades (ranks) may be appplied at once, upgrades are mutually exclusive—you may only pick one of three possible upgrades for an ability, each of which is typically a powerful variant on the base ability.