Every martial art has people who have had their influence on the creation of the martial art. Some of them are very hard to trace, for they are ancient styles which have been around for millenia. However, more recent martial arts, particularly those which originated pre World War II, or after the war, are close enough that we can tell who and what helped influence the creation of the said martial art.
Aikido is a pre-war martial art which originated in Japan, through the practices and teachings of Morihei Ueshiba. But, was he the only one who helped shape aikido? Absolutely not, here is a list of all the people who helped shape aikido, other than the founder himself.
Takeda Sōkaku was the founder/restorer of the Daito-ryu Jujutsu, a master of both physical and mental training. His most famous student was, of course, Morihei Ueshiba, the founder of aikido. Some of the most basic moves in aikido come from Daito-ryu, and Takeda is responsible for that. Daito-ryu has hundreds of techniques, separated into volumes which require linear progression from the student. Some of the pre-war aikido techniques come from Daito-ryu, and are more rigorous than the post-war techniques. Takeda Sōkaku was responsible for that, as he was master to Ueshiba.
The spiritual side of aikido was found in Onisaburo Deguchi and his teachings, which Ueshiba was fond of, to say the least.
The Ōmoto religion was what influenced Ueshiba, as well as Deguchi, who was one of the spiritual leaders of the religions. It is a shinto religion, or rather, a neo-shinto religion. Deguchi believed that the world would proceed the best through harmony and people who would create more unified, rather than on their own. He was known to be a rather happy figure and one who would often do interesting things such as dress up as Buddhist or Hindu deities or figures of importance. People in the West know him as the spiritual friend and guide to Morihei Ueshiba, the founder of aikido.
Minoru Mochizuki was a martial artist, a master of many disciplines, kendo, karate, aikido, jujutsu and many more. He was a student of aikido under Ueshiba, but he decided to travel abroad and bring aikido with him. He was responsible for helping aikido spread to the West, and it all started in France in 1950.
However, Mochizuki did not stop there.
While he brought the original aikido, the roots of aikikai, to France, he also brought his own school of aikido, called the Yoseikan. In his school, practitioners were taught to strike, as well as to defend from locks and to implement moves from judo.
Tomiki was a professor and expert in martial arts such as aikido and judo. He studied under Ueshiba, starting from 1926. In 1958, at the Waseda University, Shodokan aikido or sport aikido was created, though it was not named thusly until 1976. Tomiki earned more than a couple of frowns from the aikikai for bringing the name aikido to sports and competitions, but such is the price of popularity.
Tomiki was probably the most influential person because he brought aikido to the world and got people familiar with it, even at the price of disapproval from the Ueshiba family and the aikikai.
These people, alongside the founder of aikido, Morihei Ueshiba, were the ones to shape and mold aikido into what we know today.