The way Dragon Ball is written leaves the series with a vast amount of lore, mythology and history that's heavily implied but rarely elaborated on. Fans are instead left to speculate about certain elements of the series that are only meant to serve as background information. One of the biggest mysteries left behind by Toriyama is the true nature of the Guardian of Earth who precedes Kami.
It's tricky talking about a character with no real presence in the canon story, but there are some confirmed details and factoids about the Kami of the Previous Generation. In Japanese, the title of Guardian, Kamisama, literally translates to God. Centuries before the official start of the Dragon Ball story, the Previous Guardian acknowledges Kami. He only does so, however, after Kami removes the evil from his body, which goes on to become Demon King Piccolo. This sounds like little to go on, but several details can be inferred from what's been stated.
The first thing of note is that the actual translation for Kami or Kamisama is God. The change to Guardian of Earth is likely a deliberate mistranslation, as the name change avoids the risks of including religious connotations in a series presented to American kids. The change of Kamisama to Kami, going from a title to a name, is likely another result of this decision. The fact that the Guardian is supposed to be God speaks of the power, responsibility and sense of justice necessary for the position.
Other qualities of the Previous Guardian can be inferred by his potential similarities to Kami. The Previous Guardian's acknowledgment of Kami as his successor suggests that the two of them possess similar qualifications. When Kami is first introduced, he is presented as pure of heart and the most powerful being on Earth. Assuming the conditions for eligibility hold true for all Guardian candidates, the Previous Guardian must have been the most benevolent and powerful being on Earth in his time.
The idea that the title of Guardian is based on a mix of power and ethics is further backed by Kami's decision on who should succeed him. By beating Piccolo Jr. and winning the 23rd World Martial Arts Tournament, Goku showcases his own sense of good and power that surpasses Kami. It's for these primary reasons that Kami acknowledges Goku as a worthy successor. Goku turns down this chance, but the offer alone suggests that the Previous Guardian made a similar decision about Kami.
Any further speculation about the Previous Guardian requires the acknowledgment of The Dead Zone and the Garlic Jr. Saga as canon. According to these parts of Dragon Ball, Kami competes with another Guardian candidate named Garlic. The Previous Guardian chooses Kami over Garlic because Garlic was clearly evil. Kami, or rather the Nameless Namekian, only gets the job because he removes the evil from within himself. This strict necessity for a good heart shows both the Previous Guardian's strong sense of justice and how seriously he takes his job.
The Guardian's desire for a worthwhile successor is further shown in the Garlic Jr. Saga. When Kami tries to retrieve the Sacred Water necessary to stop Garlic Jr.'s plans, the Previous Guardian impedes his progress. He and the other former Guardians only allow Kami safe passage when he proves himself a proper Guardian. The Previous Guardian, even in death, cannot stress enough the importance of having a qualified Guardian of Earth.
While the Previous Guardian of Earth may never be fully fleshed out in canon, there are plenty of hints to what kind of a person he was. He shows signs of being a dutiful, just and benevolent figure who watched over the Earth with great vigilance. Even in death, he makes sure that whoever succeeds him is at least comparable in strength and virtue to himself. The title of Guardian of Earth is truly not one to be taken lightly by anybody seeking it.