Leo was an ancient ancestor of Lion-O. He once led the armies of Mumm-Ra, but saw his evil and formed a rebellion.
Leo lived during the rule of Mumm-Ra, at that time the Thunderians willingly served the demonic sorcerer, and Leo was Mumm-Ra's 2nd in command, during the search for the War Stone, Leo saw the evil of Mumm-Ra first hand, and the plight of the other animal people, along with Panthera, Rezard the leader of the lizards. and Shen the leader of the Jackals, Leo formed a secret rebellion against Mumm-Ra.
Eventually the War Stone was found, but Leo managed to get it and the spirit stone away from Mumm-Ra and used their combined might to defeat the Everliving, who then sealed himself within his sarcophagus where he would remain in stasis until centuries later when he was freed by Grune.
Lion-O relived these events through Leo's eyes when he was transported within The Book of Omens,
Leo was a new character created for the 2011 ThunderCats series and did not appear in the original 1980's series.
He was first seen in a flashback in the episode Journey to the Tower of Omens alongside Tygus he had the Book of Omens hidden to prevent it from falling into the wrong hands, in the following episode, Legacy where Lion-O relived a part of his life first hand, he was depicted as resembling an older version of Lion-O, except when he had possession of all the stones of power, at which point he was surrounded by a powerful suit of magical armor known as the Armor of Omens.