Sentinels

Description
Sentinels are tall (~3 1/2 times Enu's height), bipedal, presumably sentient Automatons that inhabit the world in the Bargaining stage. They help Enu through impassable terrain and the spatial manipulation puzzles. While they cannot speak, they do display signs of having emotion (Enu's Sentinel "screaming" upon seeing the corpses of other Sentinel). In Depression, it is revealed that the Sentinels can open the large, golden gates within the ruins. While there is no gameplay evidence of this, they also appear to have a Core similar to the glowing orbs dispensed by the Fish Statues.

Purpose
In Bargaining, the Sentinels serve as guides, leading Enu to his destinations. Their legs power through destructible terrain, and allow them to ascend ledges Enu himself cannot reach.

In Depression, the Sentinels are used to open the golden gates within the ruins. The Sentinels must sacrifice themselves to open their respective gates (This is how Enu's Sentinel "dies.")

Symbolism

 * In an interview with Eurogamer: "It's this reduced visibility in the game which masks the fox running off and the sentinels sacrificing themselves to open doors for you. It is as in life: "Most people when they are depressed don't notice the people who are helping - they feel they are being taken advantage and no one understands them - and that's sad, because these people are the ones who are really caring for you but of course you can't see it," [Rubio] says. "Sometimes your best friends need to sacrifice so you can move forward." "
 * It is also interesting to note how Enu creates a sentinel himself with materials from a tree (symbolic for life in many contexts) and thus one can loosely consider the sentinel as his "child." Looking at the dynamic of Enu and the Sentinel through this lense really lends a lot to the idea of Enu actually representing his father throughout the game, and is further brought home by the moment where the Sentinel sacrifices his life for Enu to pass, despite the Sentinel being the younger of the two.

Trivia

 * It is revealed in the Dev Diary that the two main Sentinels you encounter in the game, the broken down one that dies before you and the one you construct yourself, have the names Tango and Cash. Though, it isn't specified which is which.