Mickey Mouse was created in 1928 as a replacement for Oswald the Lucky Rabbit. The first two Mickey shorts drew little to none attention. And then, they release Steamboat Willie, supposed to be a parody of Steamboat Bill. It was the first animation that features synchronized music and sound effects. Steamboat Willie premiered in New York in November 1928 and became an instant hit. Following that success, a series of Mickey Mouse short films were released within months and quickly followed by a line of Mickey merchandise and Mickey Mouse Club.
On the first creation of Mickey Mouse, he isn't even called Mickey. They named him Mortimer. There are a lot of theories and speculation of why they changed the name, but one of the most popular is that Disney's wife doesn't like the name Mortimer and proposes a cuter and catchier name: Mickey. Now we know him as Mickey Mouse, but he has a middle name, Theodore. So Mickey Mouse's full name is Mickey Theodore Mouse.
Based on Wikipedia, a celebrity is a condition of fame and broad public recognition of an individual or group, or occasionally a character or animal, as a result of the attention given to them by mass media. Celebrity usually implies a favorable public image, loved by the media, as opposed to the neutral famous or simply notable. In this case, Mickey Mouse meets all the requirements to be called a celebrity. He is a very popular character and loved by most people. On top of that, he also has his own star in the Hollywood Walk of Fame.
Mickey has a lot of friends. To decide which one is the best friend is not that simple. For example, Mickey and Donald Duck are friends but they have a love-hate relationship for Mickey often making fun of Donald. Every time Mickey does nice things to Minnie, he always asks Daisy for what she gets from Donald? He does it in a jokingly way, of course. And then there's Goofy. He's a good friend of Mickey but not as close as Pluto. Yes, Pluto is a pet, but a pet can be everyone's best friend, right? Based on that, Pluto is Mickey's best friend followed by Donald and then Goofy.