Hei, Yin, Huang and Mao group surely would be in.
Then we have the main characters of each faction: Misaki Kirihara, November 11(Jack Simon), Amber and Pai
.. then secondary main character of each faction and important contractors: Kanami Ishizaki (Misaki's friend who once in love with Hei), April, Amagiri,Wei Zhijun (blood), Havoc, Alma (the continuously aging contractor, i quite like her character development),
...other main contractors: Brita(teleport girl), Mai Kashiwagi, Nick Hillman(love space travelling with his sister), Maki (young boy who wish Amber use more power so he can fit her), July, Bertha, Itzhak,Shihoko Kishida (Huang lover), Stargazer-sama
Bad guys: Yoshimitsu Hōrai (Misaki's boss), Decade (November 11's superior), Eric Nishijima (PANDORA)
Can check wikipedia for more detail but yeah the list would be long if we try to fit all of them in ( maybe we should leave Gai Kurusawa and Kiko Kayanuma out since everyone hate this duo
)