Eggman Nega

Dr. Eggman Nega, also known as Dr. Nega, is a recurring villain from the  franchise. He is the direct descendant of Dr. Eggman (though he initially claimed to be from another dimension). Silver the Hedgehog and Blaze the Cat are his main nemeses, paralleling Sonic the Hedgehog and Miles "Tails" Prower's negative relationship with Dr. Eggman.

Mario & Sonic at the Olympic Winter Games
Eggman Nega appears as a rival in the Festival Mode of the Wii version of Mario & Sonic at the Olympic Winter Games. He is faced in the Ski Cross event.

In the DS version Eggman Nega appears in the Adventure Tours mode and is faced in the Nordic Combined and Skeleton events, in Frostown and Polastraits respectively. He keeps Sparky in a cage in Frostown, and later does the same to Cuby in Polastraits.

Mario & Sonic at the London 2012 Olympic Games
Eggman Nega returns as a rival in the Wii version of Mario & Sonic at the London 2012 Olympic Games. He appears as an NPC in the London Party mode and must be spoken to by the player in order to be versed. In the first match he is faced in the Fencing event, but subsequent rematches pit him and the players in Dream Fencing events.

Eggman Nega appears in the 3DS version as a rival in the 4x100m Relay event.

Mario & Sonic at the Sochi 2014 Olympic Winter Games
Eggman Nega appears as a boss in the Legends Showdown mode of Mario & Sonic at the Sochi 2014 Olympic Winter Games. As all of the playable characters are about to claim the Legend Trophy, he ambushes them along with Dry Bowser and Rouge the Bat. The player must then face all three of them in the Winter Sports Champion Race event.

Mario & Sonic at the Rio 2016 Olympic Games
Eggman Nega appears as a playable character in this game. This not only marks his first playable appearance in the Mario & Sonic series, but also his first playable appearance in any game whatsoever.

However, Eggman Nega is only playable in the Nintendo 3DS version, and, like Espio, he only participates in the Long Jump and Long Jump Plus events. He is also unlockable, being unlocked by beating him in the Long Jump Plus event in the Road to Rio mode.

Mario & Sonic at the Olympic Games Tokyo 2020
Eggman Nega appears in Mario & Sonic at the Olympic Games Tokyo 2020, though his role in the game is currently unknown.