El Tigre I, or referred to as Mighty El Tigre by his successor, is the original El Tigre and the eldest Rivera to have lived. He is the father of Dark Leopard and Manny's great-great-great-great-great grandfather. He looked like Manny as a child and likewise could not decide whether to be a hero or a villain. The indecision drove him insane, causing him to develop a split personality. He possesses the same powers as Manny, as well as a few new ones, such as a super powerful roar and the ability to extend his claws to extreme lengths.
His El Tigre attire differs little from that of Manny's own super hero/villain persona, what with his red scarf having become a torn, old cape of sorts. He is tall and muscly, as well as sporting tattoos like blue stripes on his arms. When he opens his mouth, a pair of tiger-like fangs in his upper jaw can be seen.
El Tigre I because of his crazy condition shifts between being a good-natured guy to a maniacal laughing evil person. But he also can manage to have a normal personality to help his descendants during the episode The Grave Escape.
He was presumably in a very similar situation Manny was in his time. It is unknown if his family, friends, or enemies were anything similar to Manny's life. But the decision to be either good or evil made him crazy. He apparently remained crazy even after his death.
The Grave Escape
Manny seems to be the only person that the original El Tigre relates with the most, as both share the same powers, superpowered alter-ego and irresolution towards choosing between good and evil. While he acted with hostility towards the boy at first, he quickly softened -clearly displaying his sudden personality shifts- and even showed him some extra El Tigre powers.
The Other Riveras
Aside from Manny, El Tigre I doesn't appear to get along with the rest of the Riveras -or the rest of the inhabitants of the Land of the Dead for that matter- as he dwells in a secluded and precarious shack away from the main town. As mentioned by Golden Leon, they probably all consider him as nothing more than "babbling fool". In Miracle City however, Rodolfo Rivera and Granpapi Rivera hold him in high regard, since they place a framed picture of him every the Day of the Dead. After the episode The Grave Escape, the late Riveras managed to overcome their differences and returned together to the Land of the Dead.