Ok, I've see bias from some people since I've joined. Out of respect I'm not tagging, but just hear me out
First off, people say that all Sonic is in Smash bros is a guest. Far from true. He is a guest to begin with, but a guest is not what he is limited to. Most unlike other third party characters such as MegaMan, Snake, or PacMan, Sonic has a great history with Nintendo, countless Sonic games have been made Nintendo Exclusive, and he and his friends have been in many crossovers with Mario, plus Nintendo has a very high chance of buying SEGA. If anything, Sonic is the LEAST of a guest out of the third party characters, and so saying deserves to bring AT LEAST ONE friend along.
And no, contrary to what Sakurai claims, Sonic was not put in by fan desire alone. Remember: Sakurai is famous for not telling the whole truth. If you want to have a valid knowledge about third party involvement in a crossover like Smash, you must research the topic from the third party company as well, not just Sakurai. I've done this millions of times before, and I've learned some pretty interesting facts about it. In an interview that GameInformer did with former SEGA chairman Yuji Naka, Sonic was originally planned to start Smash in Melee. Naka asked Sakurai to add Sonic, but he could not finish working on Sonic without delaying the game, so he was put off. When working on Brawl, Sakurai remembered Naka's request and went back to SEGA and asked them if they still wanted Sonic playable. They were very reluctant at first, so the only thing that fan desire did was make SEGA's new chairman realize that it would be a good idea. And now he's one of the first veterans announced. Sakurai must like him.