The duo first met when they were eight years old, playing football in Brazil, and have since gone on to play together for Chelsea and their national team.
And Willian says they're both delighted to have reunited in north London.
"He's very happy," the forward told Arsenal Digital. "We played together in the national team, we played together at Chelsea, and now at Arsenal again. I think we can do great things together, I hope so, but I'm very happy to be with him again.
"Of course [he helped in the past], many times. Many times when I had difficult times at Chelsea he was helping me a lot. He is this kind of person that wants to help, especially young players without experience. He is the guy that helps everyone."