Del Grant was a mercenary, responsible for a revolution on Arcos and called in by Cauder on Albian to overthrow the Federation's control of the planet. The garrison was defeated under his command of the rebels, but not before the solium radiation device was activated by Tronos. When Blake arrived, Grant recognised Avon instantly. Avon believed until then that Grant was dead (Grant also believed that Avon had himself died). Having some knowledge of timing devices, he worked with Avon to disarm the solium device.
Del was Anna Grant's brother, and believed not only that she was dead but also that Avon was, albeit indirectly, responsible, having left with the exit visas before he knew Anna was dead. He appeared to have a very anti-Federation attitude, and was thus perhaps not completely mercenary. He did demand a high fee for his services, but apparently because his reputation (and ego) demanded it.