Cassiel is Shade's father and Ariel's mate. He had been banded by humans, and also went missing before Shade was born, presumed to be killed by the owls. He was actually trapped in the human-made building that Shade and his team travel to in Sunwing. He was then attached to an exploding band and brought to South America, as a tool used by the humans to bomb enemy buildings. Cassiel survived, however, along with others who stayed at Statue Haven. Before Shade came, however, he had been captured by the Vampyrum Spectrum as an intended sacrifice to Cama Zotz.

While looking over the prisoners, Goth noticed the shocking resemblance to Shade that Cassiel possessed, actually believing for a moment that it was Shade. After Shade started mass chaos and rebellion in the pyramid, Shade found Cassiel. They had a moment of disbelief and bliss before Goth came after Shade again. When the pyramid exploded, Cassiel did make it out and returned to Hibernaculum with the others. In Firewing, he and Shade share a conversation in broad daylight, since their banishment was no longer in order, due to their actions in Sunwing. Overall, Cassiel plays a large part in the Silverwing series, but is not a major character.