The steadfast Veteran Sergeant of the Dark Angels Third Company, Reuben Cassiel is a senior member of The Lion's Gaze and a powerful combatant, and servant of many battles. He recruited a young Samson Sinclaire into the ranks of the Dark Angels when The Rock was in orbit over New Sirius, looking for the initiates Tanis Barachiel had predicted would be taken from the Hive World. He found a Neophyte in Samson, and took him under his wing, teaching him the ways of the Astartes and eventually joining him on his adventures with the Lion's Gaze.
Reuben has been described as a man "as much a man of heart as he is of strategy", intuitive and observational. Having been stationed with the Dark Angels Third Company for many years, he is amongst one of the older members of The Lion's Gaze. However, Cassiel has refused to elevate to the position of Company Master for his four centuries of service, preferring to stay at a lower position and serve someone else instead, showing his characteristic modesty. He has also been remarked as a great mentor and teacher, but prone to prolonged silences and contemplations.