King Sejong the Great said in his record that the hangeul he made was based on ancient letters.
Hangeul cannot be derived from Chinese charactersbecause it's a sound letter.
About the mystery of what those ancient letters can be, some of the old forms are found across old Gojoseon territory (Manchuria etc.,) and even to Japan where old Korean ancestors must have gone across. Details are still a mystery though I blame in some part, Japanese of the ealry 20th century for burning all the books that goes beyond Joseon and Goryeo Dynasty just to make sure our history is "shorter" than theirs.
(Some 200,000 - 300,000 books were burned, taken from libraries and private homes -- the fact that Japan burnt their own ancient books that had to do with Joseon and Goryeo hardly helped).
One thing that Korea feels annoyed about Japan is not only because of what they did during WWII but having to still face the prejudice outside that Korea was influenced by Japan since the ancient times when it was the other way around.
the commonly accepted theory is that hangeul was created just by the shape of mouth,tung. that's also why world liguists think the hangeul is the best letter. all this scientific system is explained in the book "hunminjungum" written by king sejong. hangeul was not affected by old gosjosun letter"garimto" or anything. hangeul is the only letter in the world which was just made by linguists. that's why world call hangeul scientific letter.
and every chosun kings learned hangeul, becuz they should communicate with women and common people. about 18,19 century hangeul novels were popular. but in japanese regime illeteracy rate was 78 %(surveyed in 1948).it was higher than those of colonies in africa. and the word "dongyi" contains manchurians and japanese, yes also. the difference is between japanese and koreans is that japananese race contains polynesian blood at least 30 percents. but old koreans conquered japanese natives and built japanese kingdoms, so language is similar(even though alpabet letters re totally different.), it's like the relation french and english language. The difference is between manchurian and koreans is that language is not so similar as the relation of japanese and korean(even thogh maybe both re same language family).but race is similar.it's becuz manchuria is just next of korean peninsula. the ching dynasty of china was manchurians from yalu river(the northern border of korea).the ancestor of ching dynasty was from korean "hambo".