Hapkido founder Choi Yong-sool (November 9, 1904 – June 15, 1986) was abducted from his Korean home-village in south Korean peninsula at age of eight in early years of Japanese occupation of Korea. He returned back to his home-country some 30+ years later - after defeat of Japan in WWII.
What happen in-between this two events is subject of disputes. It’s quiet understandable: slaves - and Koreans, small and big, brought to Japan by occupiers were not more then slaves - don’t have biography... As Choi Yong-sool returned to the freedom and to his home-country some time between 1945 and 1948 he sure presented his life-story in better light than it really was. I can’t blame him for this, but just have to admire - he didn’t play a victim, but took his life into his own hands and did the best out of what he had.
Even his life-story differ, the more certain thing that he did spend long years with Takeda Sokaku, the founder of Daitō-ryū Aiki-jūjutsu. Words of Choi Yong-sool that he was adapted as a son by Takeda is more likely just his wishful thinking. But been taking into Takeda’s household at age of 11 after three childhood years on the streets and in temporary shelters of a hostile country, which language he didn’t master yet, he may found a father-figure in his famous, but illiberal, master.
The most reasonable accounts placing Choi besides Takeda as his male-servant. Did Takeda teach Choi his art? I doubt so - by all known accounts Takeda was chauvinistic self-centered man stranger to philanthropy. Why he wold teach his low-life Korean servant? But Choi learned it anyway - he was around all the time helping in the dojo and, maybe, at times was used as a “punch bag”. He was around much longer than any other Takeda students (including O’Sensei Morihei Ueshiba, founder of Aikido, who spend altogether just six years with Takeda), he had the skills and the fighting spirit (he would not survive if he wouldn’t). Anyway, by many accounts (including memories of Dr. Kano, founder of Judo) teaching of martial arts at this time was not how we know it on the West now: teacher just showed what to do and questions ware not accepted. So, servant in the dogo saw the same things as any student.
By some accounts in the later years Takeda accepted the fact that his male-servant learned his art better that his paying students. He even put Choi forward for the challengers who came to his dojo (at least in part, I believe, to humiliate them).
As Choi became a free man in his 40th he found that he can do good out of his fighting skills that he learned in his servitude. A few years later his art got name “Hapkido”.
So, Choi’s Hapkido is the sister-art to Ueshiba’s Aikido as both came out of Takeda’s Daitō-ryū Aiki-jūjutsu. But it’s the only thing that relate Hapkido to Aikido - in creation of Aikido O’Sensei added strong pacifistic religious concept, when Choi had no luxury nor personal history to go into philosophy and his art retained aggressive nature of jujutsu straitened by influence of Shotokan karate that about the same years got to Korea and became Takwando (presence of foreign for Aiki-jūjutsu Shotokan techniques is the reason why at times Hapkido referred as a synthetic art).