Darrius

Darrius was an elderly scientist who experimented on the plants in a jungle on Marinus, increasing their growth rate and making them more aggressive. He kept one of the keys to the Conscience of Marinus and made a fake key to fool anyone who tried to take it.

Darrius set several traps to keep the Voords away from the real key. He initially thought Barbara Wright and Ian Chesterton were Voords, until Ian saved him from a creeping plant.

Darrius, dying, told Ian where he had hidden the real key, "D-E-3-O-2", though both Ian and Barbara were confused as to what this meant. (TV: The Keys of Marinus)

Behind the scenes
Darrius was never referred to by name on-screen, only in the credits. He is also unnamed in Philip Hinchcliffe's novelisation of the story, being referred to instead as simply 'the old man'.