Nintendo

Nintendo was formerly Sega's rival. The mascot is Mario, Sonic's rival.

Competition against Sega
Nintendo had made a console called the Super Nintendo Entertainment System or SNES for short. To compete with this, Sega made a Sega Genesis. To compete against the plumber named Mario, Sega developed characters to be their mascot. Sonic the Hedgehog was later chosen to advertise Sega. They fought to get more money. Sega lost.

Sega working for Nintendo
After the Sega Dreamcast was discontinued, Sega became a third party company. The Dreamcast games Sonic Adventure and its sequel were later remade for the Nintendo Gamecube as Sonic Adventure DX: Director's Cut and Sonic Adventure 2: Battle. Sonic and the Secret Rings was later released on the Wii, along with other Sonic games.

Crossovers
Sonic and his friends competed in the Beijing 2008 Olympics against Mario characters.

Sonic was the most requested character for the Nintendo fighting game Super Smash Bros. Brawl. After the olympics, Nintendo decided to put him in. It was breaking news for the fans, which was even more hype than Solid Snake's inclusion in Brawl. Because Sonic was included late in development, his role in the Subspace Emissary was just a small cameo at the end. Mario and Sonic later competed again in the Vancouver 2010 Olympics in Mario & Sonic at the Olympic Winter Games with 4 new characters: Metal Sonic, Bowser Jr., Donkey Kong and Silver.