The Supremo was the leader of the Kaled nation in the time period prior to the creation of the Daleks. He offered Davros the opportunity to take part in a mission to destroy a new Thal weapon development facility in exchange for career advancement. (BFD: Purity)
The Supremo came to fear Davros, and ordered Fenn to destroy his work and hurt Lady Calcula. When Davros learned of this and confronted the Supremo, he coerced him to have the grant complete autonomy to the Scientific Elite. When Davros was later crippled in a Thal attack, he hoped that Davros would use a poison injector to kill himself. (BFD: Corruption)
The Supremo monitored Nyder's mission to rescue Davros when he was captured by Colonel Murash. After Davros recovered, he rejected the scientist's request that all Kaled children be handed over for genetic experimentation in an attempt to save the Kaled race. On receiving this news, Davros had the Supremo and the Council exterminated. (BFD: Guilt)
Other information
As head of the Allliance, the Doctor also went by the title of the Supremo. (PDA: Warmonger)