Added:If you're doing second characters for series like Street Fighter, there's probably a cavalcade of Sonic characters that would qualify for SEGA. At least Tails, Knuckles, and Eggman. Kiryu from Yakuza would also be another good SEGA option.
Capcom:
Jill Valentine / Leon Kennedy / Chris Redfield (Resident Evil)
Hunter (Monster Hunter)
Dante (Devil May Cry)
Phoenix Wright (Ace Attorney)
Amaterasu (Okami)
Viewtiful Joe (Viewtiful Joe)
Morrigan (Darkstalkers)
Arthur (Ghost 'n' Goblins)
Chun-Li (Street Fighter)
Square Enix:
Lara Croft (Tomb Rider)
Sora (Kingdom Hearts)
Geno (Super Mario RPG)
Bub & Bob (Bubble Bobble)
2B (Nier: Automata)
Konami:
Pyramid Head (Silent Hill)
Goemon (Mystical Ninja)
Bill Rizer / Lance Bean (Contra)
SEGA:
Tails / Knuckles / Eggman (Sonic)
Kiryu (Yakuza)
Marvelous:
Asuka (Senran Kagura)
Farmer (Harvest Moon)
Adventurer (Rune Factory)
SNK:
Terry Bogard / Mai Shiranui (The King of Fighters)
Koei Tecmo:
Ryu Hayabusa (Ninja Gaiden)
Kasumi (Dead or Alive)
Bandai Namco:
Lloyd Irving (Tales of Symphonia)
Heihachi / Jin Kazama / Kazuya Mishima (Tekken)
Nightmare (Soulcalibur)
Yoshimitsu (Tekken / Soulcalibur)
KOS-MOS (Xenosaga)
Agumon (Digimon)
Solaire / Artorias (Dark Souls)
Level-5:
Professor Layton (Professor Layton)
Jibanyan (Yo-kai Watch)
Spike Chunsoft:
Shiren (Mystery Dungeon)
Nihon Falcom:
Adol Christin (Ys series)
Team Shanghai Alice
Reimu Hakurei (Touhou Project)
Grasshopper:
Travis Touchdown (No More Heroes)
Ubisoft:
Ezio Auditore (Assassin's Creed)
Rayman (Rayman)
Prince (Prince of Persia)
Sam Fisher (Splinter Cell)
Microsoft:
Master Chief (Halo)
Steve (Minecraft)
Activision / Blizzard:
Spyro (Spyro)
Crash (Crash Bandicoot)
Diablo (Diablo)
Arthas (World of Warcraft)
Sarah Kerrigan (Starcraft)
Tracer (Overwatch)
Bethesda:
Doomslayer (Doom)
Dovahkiin (Elder Scrolls)
Rockstar:
Max Payne (Max Payne)
Electronic Arts:
Commander Shepard (Mass Effect)
IO Interactive:
Agent 47 (Hitman)
Warner Bros:
Scorpion / Sub-Zero (Mortal Kombat)
Riot:
Ryze / Ashe / Jax / Teemo (League of Legends)
Jill Valentine / Leon Kennedy / Chris Redfield (Resident Evil)
Hunter (Monster Hunter)
Dante (Devil May Cry)
Phoenix Wright (Ace Attorney)
Amaterasu (Okami)
Viewtiful Joe (Viewtiful Joe)
Morrigan (Darkstalkers)
Arthur (Ghost 'n' Goblins)
Chun-Li (Street Fighter)
Square Enix:
Lara Croft (Tomb Rider)
Sora (Kingdom Hearts)
Geno (Super Mario RPG)
Bub & Bob (Bubble Bobble)
2B (Nier: Automata)
Konami:
Pyramid Head (Silent Hill)
Goemon (Mystical Ninja)
Bill Rizer / Lance Bean (Contra)
SEGA:
Tails / Knuckles / Eggman (Sonic)
Kiryu (Yakuza)
Marvelous:
Asuka (Senran Kagura)
Farmer (Harvest Moon)
Adventurer (Rune Factory)
SNK:
Koei Tecmo:
Ryu Hayabusa (Ninja Gaiden)
Kasumi (Dead or Alive)
Bandai Namco:
Lloyd Irving (Tales of Symphonia)
Heihachi / Jin Kazama / Kazuya Mishima (Tekken)
Nightmare (Soulcalibur)
Yoshimitsu (Tekken / Soulcalibur)
KOS-MOS (Xenosaga)
Agumon (Digimon)
Solaire / Artorias (Dark Souls)
Level-5:
Professor Layton (Professor Layton)
Jibanyan (Yo-kai Watch)
Spike Chunsoft:
Shiren (Mystery Dungeon)
Nihon Falcom:
Adol Christin (Ys series)
Team Shanghai Alice
Reimu Hakurei (Touhou Project)
Grasshopper:
Travis Touchdown (No More Heroes)
Ubisoft:
Ezio Auditore (Assassin's Creed)
Rayman (Rayman)
Prince (Prince of Persia)
Sam Fisher (Splinter Cell)
Microsoft:
Master Chief (Halo)
Steve (Minecraft)
Activision / Blizzard:
Spyro (Spyro)
Crash (Crash Bandicoot)
Diablo (Diablo)
Arthas (World of Warcraft)
Sarah Kerrigan (Starcraft)
Tracer (Overwatch)
Bethesda:
Doomslayer (Doom)
Dovahkiin (Elder Scrolls)
Rockstar:
Max Payne (Max Payne)
Electronic Arts:
Commander Shepard (Mass Effect)
IO Interactive:
Agent 47 (Hitman)
Warner Bros:
Scorpion / Sub-Zero (Mortal Kombat)
Riot:
Ryze / Ashe / Jax / Teemo (League of Legends)