This is pretty simple. The first person posts an anime name, and the second person posts the character they think is most popular in the fandom of the show, then posts another anime!
(((THE MAIN CHARACTER IS NOT ALWAYS THE MOST POPULAR! Ex: I think that Kanade from Angel Beats is more popular than the main character. )))
Also, you don't have to be worried about the character being the number one popular character of that anime. Just as long as they're well known.
I don't take credit for the picture below.
I'll go first~ Who do you think is the most popular character of Durarara?