Alex Kidd
Description:
Alex Kidd, a video game character, is a monkey-like boy with big ears in a red jumpsuit and was commonly regarded as Sega's mascot until Sonic the Hedgehog debuted in 1991.
Last seen in Alex Kidd in Shinobi World (1990)
Current status:Various cameos and collection games (Sega Superstar Tennis, Genesis Collection) with buzz about new game in development.
Bonk
Description:
Bonk is a video game character from NEC's TurboGrafx-16 console. Known in Japan as "PC-Genjin" and as "BC Kid" in PAL territories, Bonk was a mascot for NEC's console.
Last seen in Super Bonk (1995)
Current Status: Dead with various cameos such as Saturn Bomberman and Mobile phone games.
Bubsy
Description:
Bubsy is a series of platform video games released by Accolade for the SNES, Sega Mega Drive/Genesis, Atari Jaguar, the PC and PlayStation in the early and mid-1990s.
Last seen in Bubsy 3D (1997)
Current status: Dead due to catastrophic failure of Bubsy 3D
Earthworm Jim
Description: Earthworm Jim is a series of side-scrolling platforming video games. First released in 1994, the series is noted for its shooting/platforming gameplay and humorous and edgy style
Last seen in Earthworm Jim 3D(1999)
Current Status: Alive. Earthworm Jim 4 has been announced
Gex
Description: Gex is a platformer video game series, developed by Crystal Dynamics, that details the adventures of a gecko named Gex.
Last seen in Gex 3: Deep Cover Gecko (1999)
Current Status: Permanent hibernation, but chances of return to gaming do exist as both Crystal Dynamics and EDIOS still exist and own the rights to the series.
Nights
Description:
Nights is a video game character and the protagonist of the critically acclaimed Nights into Dreams... and Nights: Journey of Dreams, developed and published by Sega.
Last seen in Nights: Journey of Dreams (2007)
Curent Status: With a commercially failed sequel, looks like the end of the road for Nights. Cameos will surelly happen however.
Rayman
Description:
Rayman is an anthropomorphic video game character, and is the official mascot of the video game publisher Ubisoft.
Last seen in Rayman: Hoodlums' Revenge (2005)
Curent status: Alive but no major game announcement.
Shinobi
Description: Along with Alex Kidd and Sonic the Hedgehog, the Shinobi ninja has long been one of Sega's flagship characters, acting as a mascot for a short time in the late 1980's when the ninja boom was in full force. Its games are a showcase of Sega's technical accomplishment. Last seen in Nightshade (2004) Curent status: Ninjas never die...
Ulala (Space Channel 5)
Description:
Ulala is a character in the Space Channel 5 series developed by United Game Artists under the direction of Tetsuya Mizuguchi and published by Sega.
Last seen in Space Channel 5: Ulala's Cosmic Attack (2003)
Curent status: Alive with cameos but missing in action. However...
"According to an article on Eurogamer.net, several "Higher Ups" at Sega are pushing for a new SC5 game to be made for a next-generation console. However, producer Takashi Yuda says that "nothing has been decided."Series producer Tetsuya Mizuguchi has expressed interest in another Space Channel 5 sequel for the Nintendo Wii, although no plans appear to be in the works at this time."
http://en.wikipedia.org/wiki/Space_Channel_5#Future_of_the_series
Vectorman
Description:
Vectorman is a shooter platform game developed by BlueSky Software and published by Sega for the Sega Mega Drive/Genesis. Vectorman is both a critical and commercial success, becoming a Genesis Mega hit.
Last seen in Vectorman 2 (1996)
Current status: Dead. Game announcement in 2003 was canceled. Various Genesis collections...
Zool
Description:
Zool is a British computer game originally produced for the Amiga by Gremlin Graphics as a rival to Sega's Sonic the Hedgehog.
Last seen in Zool 2 (1994)
Current status: Dead with Amiga/Commodore collapsing in 1995.
Make other suggestions and I'll add more... ;)