Kati Sol Caesar is the step brother of Princess Piña, Prince Zorzal and Prince Diabo. He is also the step son of Emperor Molt Sol Augustus.

In the Gaiden we are told that he is the son of the prior Emperor, Grahn, who had died young. Molt was ordered to adopt him by the Senate until he came of age and could have the throne transferred over. While Molt followed the first part, he had Kati killed to ensure that he could keep the royal line for himself. This is never made clear to anyone else, and even Zorzal claims that he was "pretending to be a fool", only to avoid being killed as Kati was, by his own father.

Before he died, Kati had a daughter with Corona who later married Duke Prended Sol Randall and raised her as Lady Fre Randall. This information was only revealed to her on the Duke's deathbed.