Katakana is simplified from the regular script of Chinese characters in China, which was invented in the early Heian period and used for training and reading chinese. But the current katakana glyphs were determined during the Meiji period. Before that, a pronunciation often had multiple katakana corresponding to it.
Early textbooks in Taiwan Province claimed that Katakana was created by Kibi No Asomi Makibi, but this is not true. The cause of pseudonym is the simplification of Chinese characters, not the work of one person.