Nendo is basically Saiki's sidekick. He's not very smart, and he thinks Saiki is his best friend, even though Saiki doesn't like him. Saiki also can't read Nendo's mind. I belive yare yare means "Oh my," or something along those lines.
Answer:
An adult is a mature, fully developed person. An adult has reached the age when they are legally responsible for their actions.
After Mr. Bates passed away, Gulliver's business began to fail. Thus, he decided to go back to sea and work as a ship's surgeon. This is why he originally went to sea.
However, at the end of book 1, Gulliver accidentally meets a ship going back to England and decides to go back to his native country.
