Brother Captain Cestius Gallus
Brother Captain Cestius Gallus of the XIII Legion (Ultramarines)
Captain Cestius Gallus of the Ultramarines is the highest ranking Astartes onboard Terminal 509.
He and his retinue has recently visited Terra to present to the Museum of Victories a relic recovered by Roboute Guilliman himself, the Primarch of the Ultramarines. They are now awaiting transit clearance for their return to Ultramar to join the muster at Calth.
Update: after the aborted sabotage attempt by Gorek & Sekesh of the Word Bearers Legion, Captain Gallus assumed command of all Astartes on Terminal 509.
He recently dispatched a small team of Astartes operatives to Populonia to trace the origin of the Navy frigate that attempted to destroy Terminal 509.