I would say that there are four kinds of videogame sidekicks: the Guide, the Voice, the Princess and the Silent One . The Guide explains the game world to the player and also stands for the absolute truth for him, what can be important for gameplay and puzzles. The Voice gives the silent character an voice for dialogs and other interactions, gives the main characters someone to talk about his thoughts (Internal monologue) and entertains the player in lonely places. The Princess is the characters for escort missions, which the player needs to protect for danger. Finally the Silent One is mostly a gameplay feature, which needs to be fast to use. That's why he almost never talks and is often an animal to order around.
The important part is, that the sidekick is an interesting character with deepness, usefulness or enjoyment to fill a long journey and builds a importants to the player, so that he wants to protect him/her. If this fails, which happens especially to the Princess, people begin to create big hade against them. Who wants to hang around with a person, who is not funny, useless and annoying.
Best example is to compare Navi from Ocerina of Time and Tatl from Majors Mask. Tatl gets almost no hate in comparison to Navi. Even so they look and work the same. But the difference is, that Tatl was an much more interesting character. She had flaws, goals, opinions and importantly an arc through the game. On the other hand Navi hasn't much going for her. She yelled some times, but hasn't good personalty.