Uka Uka is the evil younger twin brother of Aku Aku. During his physical life, Uka Uka was committed to a life of skulduggery.
To protect the world from his malice, Aku Aku imprisoned Uka Uka in an underground temple prison, where he would be incarcerated for many an eon. When Cortex's space station is sent crashing to earth in the beginning of Crash Bandicoot 3: Warped, the temple is destroyed in the crash, liberating Uka Uka from his prison.
Uka Uka now supervises Doctor Cortex's plots for world domination, and though he doesn't necessarily trust Cortex due to his chronic failures, he keeps Cortex alive as a debt of gratitude for indirectly setting him free.
From Warped up to Crash Nitro Kart, Uka Uka is voiced by Clancy Brown, who also voices Doctor Cortex in the same series.
In Crash Twinsanity, Uka Uka is voiced by Alex Fernandez.
In the games starting with Crash of the Titans, he is voiced by John DiMaggio, who previously voiced Tiny in Crash Nitro Kart.