A mythical entity blamed for causing disruption and minor mischief in Dwarf mines: sometimes takes credit for alerting Dwarfs to a possible lethal working meriting the attention of the Knockermen. If Dwarfs had Gods, Agi would be their Trickster.
Agi is blamed for many randomized unwanted happenings in much the same way as humans used to blame Puck, or Robin Goodfellow.
His name is also taken by warmongers amongst the dwarfs (as in The Sons of Agi Hammerthief) in The Fifth Elephant in the same way as some jihadists take the name of Islamic warriors past (such as the original Mahdi) as their name and justification for holy war or we have the "Real" IRA taking names that they don't necessarily have the right to.
A Dwarfish saying related to this character is "Stick it where Agi put the coal," a self-translating obscenity. Sam Vimes has likened Agi to the Dwarfish equivalent of Sweet Fanny Adams.