Master Lumin is something rare - an Imperial astropath that's become a full-fledged Chaos sorcerer. That's not an unique set of skills, but it's rare enough that he's the only one with that skill set within Castor's little fleet. As an astropath Lumin can send astropathic messages to his peers (often pretending to be one of them) or listen in on any unencrypted astrotelepathic communication. As a sorcerer Lumin has the power to communicate over interstellar distances using certain rituals - and he also knows how to summon entities from the Luminous Realm (that's what he call's the Immaterium). He's also something of a diviner, using his own customized version of the Imperial Tarot to good effect. Finally Lumin has a bit of leadeship and organizational skills and he's a passable teacher; meaning he makes a pretty effective leader for the fleet's sorcerers and diviners. A valuable person indeed.