Hal Mellanby was a weapons designer and dissident from a generation before Blake's time. After a revolt he led was crushed by the Federation he fled Earth with his daughter Dayna, settling on the planet Sarran.
Mellanby believed people had the right to freedom and independence and rejected the authoritarian regime of the Federation. At one point he was taken in for interrogation by Federation security, and his optic nerve was burned out in the process. He compensated for this with a chest-mounted vision amplification device.
After this he led a major revolt against the Federation which took three weeks to crush. Ultimately the rebels surrendered - but they were executed en masse once they had laid down their arms. Only Mellanby survived - some of his supporters thought him a coward for this, as he did himself sometimes. But, given the chance to save his daughter's life he took it and they went into hiding on Sarran. For at least a while there was a three million credit bounty on his head. In seclusion, Mellanby continued to develop weapons systems and monitor Federation affairs as much as he could. He also adopted a Sarran girl, Lauren, raising her as his daughter. He followed the exploits of Roj Blake aboard the Liberator and greatly admired him.
Towards the end of the Intergalactic War, the life pods of Avon and Servalan both landed on Sarran. They were rescued by Dayna and taken to Mellanby's base. Knowing she needed Avon's help to escape, Servalan stole Orac in an attempt to gain leverage to use against him. Mellanby interrupted her in this and she shot and killed him.
Mellanby was a contradictory individual in many ways - a fierce and principled idealist, but also a devoted parent. A highly-skilled weapons designer, he was also strongly against the taking of life except when it was necessary. In later life he admitted to wondering what his motivation for leaving Earth when his attempted revolution had failed was - a desire to save his daughter's life, or simply cowardice. For all his animosity to the Federation he showed no hostility towards Servalan until her attempt to steal Orac.