Kanji came from China
Chinese coins engraved with Chinese characters were found in ancient Japanese ruins. The oldest coins made around the 1st century BC. From this point of view, the people who lived in the Japan first saw Kanji in the first century AD at the latest, considering the time it takes for money to come from China. But whether or not those who saw the Kanji recognized it as a letter is another matter. Perhaps people at that time could only see the Kanji as patterns, as we do when looking at Arabic letters, for example. So when did the Japanese anscestors start to understand Kanji as characters? It is assumed about the 3rd century AD, the time of "Yamataikoku". It is certain that there were envoys between Japan and China at that time as seen in "Gishiwajinden". It is believed that there must had been people in Yamataikoku who could understand the Kanji and handle the documents from China.