Introduction to the Player
The Guardian Angel appears to the player at the start of Borderlands, as the 4 player characters ride Marcus' bus into the town of Fyrestone. The Guardian Angel implies that you have been chosen for a quest of great importance, which turns out to be the quest to find, and ultimately seal the Eridian Vault that has caused so much conflict on Pandora.
During the Game
The Guardian Angel often appears at the beginning and end of main quests, either cautioning or counseling the player about the tasks that lie before them, or congratulating the player upon the completion of quests.
Throughout the game, the Guardian Angel praises the player for their actions and often states that the player character's actions have reinforced the Guardian Angel's faith that the Player Character was the correct choice for the task that she gave them. This praise is often the most positive that the player will receive in the game.
The Guardian Angel may have a special affinity for Claptraps, given that during one of the opening missions of the game, the Guardian Angel shows serious consternation over an injured Claptrap, and immediately tasks the player with finding a repair kit to fix the Claptrap.
The end of Borderlands strongly suggests that the Guardian Angel is an A.I, due to a camera shot that implies that the Angel's messages are being sent to the Player from a satellite. Additionally, the game suggests that the Guardian Angel can only speak to you over the ECHOnet, and while the player is using the HUD that he or she is given at the beginning of the game. However, this is contradicted by the fact that the Guardian Angel initially contacts you on Marcus's bus before you have been given your HUD. Additionally, the Guardian Angel seems to be the most knowledgeable of any character on Pandora concerning the Eridian aliens who inhabited Pandora. This evidence suggests that she is in fact an alien AI or other non-physical construct which is for some reason using seemingly modern technology to contact the player.
It is confirmed in the opening mission of Borderlands 2 that the Guardian Angel is in fact an A.I.
Relationship to 4th Borderlands DLC
The progression of shots for the ending cut scene of Borderlands may be taken to imply that the deactivation of the Guardian Angel's satellite somehow lead to the malfunction of the Claptrap that becomes the Interplanetary Ninja Assassin. In the scene where the Guardian Angel powers down, a small blue-green light on the satellite is shown that fades to black as the power goes off. Immediately after this, a Claptrap is shown, with the same blue-green eye light as was on the satellite, which then fades to red, at which point the Claptrap becomes the Interplanetary Ninja Assassin. This, along with the Guardian Angel's affinity for claptraps may suggest that she was somehow controlling the Claptraps, and that when she shut off it caused the claptraps to malfunction, leading to the Claptrap rebellion that is the focus of the 4th Borderlands DLC.
In addition, it should be noted that the satellite is a Hyperion model satellite, and Hyperion is also stated to be the makers of Claptraps in the 3rd DLC and the sequel.