Balthamos and his lover, Baruch, are both angels in rebellion from the Kingdom of Heaven. They seek to become part of Lord Asriel's army of angels and overthrow Metatron, the acting Regent of The Authority
Both angels find Will Parry at the end of The Subtle Knife; because Will is the bearer of the subtle knife, they hope to bring him to Lord Asriel. However, Will's companion Lyra Belacqua has been kidnapped and Will refuses to go to Asriel until Lyra is found. The angels agree to accompany Will without question until she is found.
This decision proves to be dangerous; both Baruch and Balthamos are aware the Regent is after them, especially now that they are being accompanied by Will. An unfortunate encounter leads to Baruch being fatally wounded. Balthamos and Will are separated from Baruch, who has gone to Lord Asriel in his wounded state. Baruch provides Asriel with all the information he and Balthamos have discovered about Will, Lyra, and the Regent before dying.
Baruch's death is immediately felt by Balthamos as a result of the strong bond of love they shared. Striken with grief, Balthamos promises to aid Will in every way possible to honour Baruch's sacrifice. When Balthamos is unable to do so, he runs away, grieving over Baruch's death and feeling guilty about abandoning Will. The angel does not appear again until the end of the book, when Balthamos confronts an assassin who was hoping to kill Lyra by tracking Dr Mary Malone. After killing him, he says he was too weak to keep living, so he lets his life fade away.
Of the two angels, Balthamos is the more passive of the pair. He admits that Baruch is the courageous one and whenever Balthamos fails at something important, Baruch is the one to help him fix things. Balthamos has a sarcastic personality, and his relations with Will are conducted with an air of ironic contempt.
As angels of low rank, Balthamos and Baruch's abilities are limited. They appear as luminous humanlike forms, which are barely visible to human eyes even at night. The angels also possess the power to transform. They are capable of flight though their wings do not have a corporeal form.
Balthamos' gender has confused readers, with many believing that Balthamos was a female in his previous life, as he states in The Subtle Knife that he was not a man previously. However, this more than likely refers to Balthamos never actually existing as a human, the term "man" is used to refer to humankind rather than gender, meaning that Balthamos has likely only ever existed as an angel, as opposed to Baruch, who was a man before he became an angel.
In the BBC radio series, Balthamos is played by Ray Fearon and serves as the series' narrator. Here, he presents himself to the listener as a "recorcing angel" and a "listener of lives" who observes events as they take place and provides description and exposition for the listener.