Out-of-universe, The Player moves Niko and manages their inventory with the keyboard; in-universe, what actually occurs is ambiguous. It is implied that The Player is not directly telling Niko what to do, nor is The Player physically controlling Niko, as both movement and inventory management are done before Niko is ever aware that The Player exists. Prophetbot remarks that The Player has been "guiding" Niko throughout the entirety of their journey since slightly after they woke up (the moment the first cutscene ends).
The Player converses often with Niko about their journey and both of their pasts. The Player has the choice of choosing conversational options that will develop their character; for example, Niko will ask The Player if they reside in a city or somewhere else.
The Player's guidance is critical to Niko's progress, as throughout their journey, they encounter several puzzles and elements outside of the game itself, which Niko cannot access and which The Player must intervene.
The Player champions themselves as a 'kind god' and a helpful god, which Niko later reflects mutually. The Player and Niko are implied to have been very good friends from the time they spent together, with The Player having the optional ability to urge Niko to perform actions such as making them do a robot voice and riding around on an automated floor vacuum.
Based on in-game dialogue and options, The Player is equally as determined as Niko to save the world. They often lend Niko their aid in puzzle solving and deciding what action to take next, acting as an adviser or strategist to Niko.
The Player has proven themselves to be very resourceful, as they are able to improvise solutions and plan for the future on-the-fly. It is implied that The Player is solely managing Niko's inventory (as Niko will remark on The Player's inventory decisions), demonstrating that The Player is able to combine and repurpose countless items to suit the cause, speeding Niko's progress through the world.
The Player is known to easily collapse under guilt, as they are completely unable to hide the truth about The Sun from Niko, even if the out-of-universe user deliberately chooses the "keep quiet" options. The World Machine even taunts The Player for their immense care for Niko by saying "You might as well tell Niko the truth. You do care about Niko, don't you?".
As evidenced by The Player's in-game title being "god" and not "God", The Player has several abilities beyond the boundaries of the game but is neither omnipotent nor omniscient. This is referenced at least once at the end of the game when The Player learns the truth about The Sun for the first time, and admits to Niko that they don't know what will happen to them.