Phantasy Star Online was the ORPG developed by part of Sonic Team for the Dreamcast. This is a photo from the lobby area of the old Dreamcast ship. It's a test server (seen in top right where the name of the ship would usually be) and this is a photo of the developers trying out the 3D character models they made for Sonic, Tails & Knuckles that work in the game. They acted as "mini mascot suits" where a developer could control them just like a real mascot suit & interact with characters. They also appeared as NPCs in a special quest in the game. If you passed the quest (Help Sonic get rings by solving fence related puzzles) you could get a prize weapon. It was the "Sonic Knuckle" which was a pair of Knuckles-like gloves your character could wear!
Sonic would even follow your character around in the area, as you switched fences on & off to try to solve the puzzles on Ragol (the planet you see out the window in the photo). They talked to you (with a script) so it was really a cool crossover & something special for the fans. Each edition of Phantasy Star (Universe & Online2) also got Sonic things for you to see.