The Vanguard, a breakaway faction of the Sentinel, want the twin worlds of Stark and Arcadia reunited, and their plan in TLJ was to do this by controlling the Guardian. They originally represented the scientists present at the Divide, the Starkian 6 of the 13 founding members of the Sentinel. Although originally faithful to the Sentinel, distance from their Arcadian brothers led to their forgetting the importance of the Balance. They grew to desire the reunion of the Two Worlds, which finally led to a schism.
The Vanguard's plan to install their own Guardian involved finding and obtaining children "born to the Balance" - i.e. Guardians-to-be - and running experiments in an attempt to control them. These experiments have largely been a failure, though one resulted in Gordon Halloway's being split into two. Gordon later became head of the Vanguard's secret police.
In Stark the Vanguard are working under the cover of the Church of Voltec: because Starkers are unaware of the Balance and the Divide they cannot recruit openly. Through this religion and the company Malkuth Technologies Incorporated they wield great power in Newport and across the world. According to Cortez, "they own multinational companies. They own planets. They own armies." When April visits the MTI building on Grendel Avenue, she notes that the company logo, a cross through a circle, probably symbolises the destruction of the Balance.
In Arcadia, April discovers that the ideas and theories of the Vanguard are starting to appeal to many of the people there. One of them is Lorhan, a captive of Roper Klacks, who scorns the idea of the Balance and the Guardian calling him a "naked guy in a tower." He says that the Vanguard are right, that "magic and science belong together, in the hands of the people". The Marcurian Cups Handler is gleeful at the prospect of a reunited Earth, whereas Karek the map seller is more circumspect, but thinks that machines, rather than Arcadian chaos and magic, are the future of mankind.
The Vanguard are powerful in Stark, where they have a black ops squad, advanced technology, genetic engineering and vast commercial interests at their disposal. In Arcadia their agents include the bellicose Tyren and the Alchemist Roper Klacks, although their recruiting tactics apparently didn't work on the Gribbler. Jacob McAllen's role is not entirely spelled out. He is old enough to have led the schism of the Vanguard and is also CEO of MTI, so he is generally seen as being head of the Vanguard by default. It is possible he took this role at a later point, however, rather than having been there from the beginning. The status of the Vanguard after the apparent demise of McAllen and redemption of Gordon is unknown. Their influence in Stark would have been severely weakened if not destroyed by the Collapse, while thier influence in Arcadia is also minimal due to Azadi exterminating the Tyren and usurping their roles as bringers of technology.
The Vanguard Agent at the Border House is voiced by Frank Rivers.