Basically, when it comes to the characters in order of importance, there are about 5 or 6 levels.
TOP level: All playable MK chars in MKII, Shaolin Monks & MKvsDC. Notice that this automatically includes all from MKI (except Goro) and MKII (except Kintaro).
- Scorpion, Sub-Zero, Reptile, Liu Kang, Kung Lao, Baraka, Johnny Cage, Kitana, Jax, Sonya, Shang Tsung, Raiden, Mileena, Kano, Shao Kahn
2nd level: All playable MK chars in MKT/MK9 (including DLC) that were in at least one other game.
- this includes Goro, Kintaro, Motaro, Rain, Kenshi, Quan Chi etc
3rd level: All remaining playable MK chars in MKA.
- this includes Taven, Daegon, Shujinko, Meat, Onaga etc
4th level: All remaining playable chars from any game in franchise, all fightable MK chars who were never playable,
- this includes Tremor, Director, ST's Zombie, Cyber Reptile from challenge tower etc, Monster from MKD, Ruutuu, Stuntman, Unit 5 etc, all chars from SM, SF, MKMSZ and A konquest like tarkatan grunts, masked guards, shadow priests, tengu ninjas, jungle trainers, Shirai Ryu undead, Wu Lae, Ice Beast, Flaming Skulls, Tasia, No Face, Kia, Jataaka, etc
LOWEST level: all background, non playable, non fightable chars.
- this includes Delia and Argus, Orin and Caro, Apep, Jerrod, One Being (oddly enough), Damashi (kinda sorta) etc
GUEST level: All other non MK chars.
- Freddy, Kratos and DCU chars (including Dark Kahn)
The ones at the top are in most games, and really are the core of the franchise. They need to 'sit back' and rest for a game before bouncing right back into the fray for another 2 or 3 rounds (entries). I think everyone gets bored of seeing the mains too often, but not often enough and it wouldn't be MK.
