Cassio Tagge is a human Imperial general from Tepasi, born into the wealthy House of Tagge, played by Don Henderson in A New Hope (1977). He is the officer who warns the Death Star conference that the Rebellion remains dangerous while the station is not yet fully operational — and who then vanishes from the film entirely.
The gap Marvel filled
Kieron Gillen noticed that disappearance. Tagge is absent from the second half of A New Hope, which means nothing established that he died at Yavin — so the 2015 Darth Vader series brought him back as Vader's institutional rival, promoted to Grand General and placed above the Sith Lord in the chain of command by an Emperor who wanted Vader humbled after losing the Death Star.
The rivalry
It is one of the most productive things anyone has done with a background extra. Tagge is competent, vindicated by events, well-connected, and fatally convinced that Vader is an asset to be managed rather than a person to be feared. That misjudgement plays out across the run and ends the way misjudgements about Vader always end: he is demoted after the Cylo affair collapses and strangled for it.
The family
House Tagge and the Tagge Corporation are a long-running fixture of Star Wars comics going back to the original Marvel run of the 1970s, where various Tagge siblings pursued a vendetta against Luke Skywalker. Cassio is the point where that comics-only aristocracy touches the films.