Angel

The Guardian Angel is a mysterious and enigmatic presence watching over the player. It speaks with the voice of a human female and projects images of a beautiful woman with black hair and piercing blue eyes, but it is unknown whether this is its actual appearance or merely projected images to appeal to the player.

Details
Cortana...I mean...The Angel watches over all of the player's actions through the ECHO-net, a satellite network for communication setup by the Dahl Corporation for communication across Pandora. The player receives an ECHO device from a Claptrap after disembarking from the bus at the beginning of the game. It seems like the Angel uses the Claptraps to guide the player through the game, since they open access to areas and the Angel is very interested in seeing them operational.

While her exact motivations are unknown, she is keenly interested in the player obtaining all of the pieces to the Vault Key and being there for the Vault's opening, which can only be done every 200 years. We do know that the Angel is not affiliated with Atlas or the Crimson Lance, since she opposed them throughout the game.

Speculation
From clues and hints dropped throughout the game, it can be concluded the Guardian Angel is not human. Instead, it seems like the Angel is an AI that is either plugged into Dahl's ECHO-net or actually onboard the satellites that manage and transmit the ECHO-net, through which it monitors and communicates with the player. However, this is unlikely as you get a transmission from her before receiving an ECHO.

Additionally, the player receives two communications from the Guardian Angel regarding the window for opening the vault by saying that "We will not have another opportunity to open the Vault in this lifetime" and "We will not have another opportunity to open the Vault in your lifetime," referring to the Vault only being able to be opened every 200 years.

Since opening the Vault releases The Destroyer, it can be assumed that either the Vault is a prison for the terrible monster or a gateway it is using to try to enter this dimension. Either way, it seems that the Angel's purpose in guiding the player to the Vault is to prevent The Destroyer from entering this world by defeating it.

Perhaps the satellite serves as a 'host' for the Guardian Angel, much like The Destroyer which apparently also needs a host to exist on a particular dimension/plane.

Possible origins
From various clues in the game, It may be concluded that the Guardian Angel could be either an AI created by Tannis either for company in her psychosis, or accidentally created when her computers were fried while she was trying to translate Eridian writings. Another possibility is that the Guardian Angel is a split personality of Tannis, which expressed itself thorough the ECHO network. Other theories postulate that the Guardian Angel is an agent of the Hyperion corporation or is actually an Eridian.

Clues that lead to this are as follows:


 * Tannis "practically invented" the ECHO network which The Guardian Angel relies on. (Contradiction: Yet she could converse to you on Marcus' bus, before you received the Echo Device from the Cl4P-TP.)
 * Tannis called the ECHO recorder "two timing" suggesting there was another being in her encampment, even though she was the only living human there.
 * Tannis asks, "Did you get my message?" when you discover her in a cell. She also says, "They were foolish letting me keep my Communicator," meaning that her alter-ego could have sent the message from the cell.
 * The paper you get from her is written in the form of an argument with an "invisible cellmate".
 * If the Guardian Angel is an AI made by Tannis, this would explain her knowledge of the Vault.
 * The Guardian Angel tells you to deliver the key to Tannis to get a reward.
 * When returning one of the Vault keys to Tannis, the Guardian Angel says in a transmission, "Congratulations! You got another piece of the key without my help! ...Or did you? (Light giggling)," after Tannis helped you find it.

Another possible, and similar theory is that the Guardian Angel was not created by Tannis, but was communicating with her just the same as with the player character and also using Tannis' computer. A comment made after defeating Krom and collecting the Vault Key Fragment in his possession suggests that the Guardian Angel had a hand in leading you to the key, despite never talking to you. Another factor that can also be explained by this, is that Tannis was able to send out a message that, despite being impossible to understand, did reach the player characters Echo Communicator, suggesting that Tannis' communicator was still fully functional, and thus, could be used by the Guardian Angel as her eyes, leading to the argument between Tannis and potentially the Angel, that Tannis wrote in her cell about reactivating the EchoNet. The Angel's ability to hear what is spoken by others is questionable at best, but as she can use the communicators as her eyes, it is very possible that the argument on the napkin was Tannis' method of replying to the Angel.

It is also possible that the Hyperion corporation, being the closest direct competitor to Atlas, used their extensive spy and espionage network to find out what they, Tannis, and the player was up to, and work to prevent it. The Guardian Angel may be an agent or AI used by Hyperion to prevent the release of the Destroyer, or to somehow further Hyperion's own ends.

An alternate theory is that the Eridians were a cybernetic race (which isn't improbable, given the sophistication of their technology and that all their weapons are electricly based and have names written in binary) and the Guardian Angel is, in fact, a surviving member of the Eridian race, tasked with keeping the Destroyer trapped in the Vault. Being a cybernetic being, the Guardian Angel would be able to interface with the Hyperion spy satellite and link into the ECHO network to watch and communicate with the player.

Trivia

 * The Guardian Angel is portrayed by actress Britanni Deniece Johnson.
 * The following videos show her transformation to the Guardian Angel, step by step.