Shiroe,
Naotsugu, and
Nyanta are the major characters who were former members of the Debauchery Tea Party, and protagonists in the anime.
Kanami was the de facto leader of the group. She was like a selfish sunshine. "Hey guys! I wanna do this let's do this today!" *leads the pack to whatever her heart desired the day* "Hey guys! I wanna go here today let's go here today!"
Energetic, positive, and knew how to hav fun. Full of charisma and talent, everyone loved her and adored her. But she had to retire from the game as she moved to overseas, which was one of the major reason for breaking up the group. She did, however, later create a new character in Rome, and became trapped in this world there.
She's a major character, but will not have any appearance other than flashbacks in the anime. She is one of the lead in the Overseas side-shot, leading
Leonardo a Turtle Ninja yankee,
Coppelia, a Chinese RMT bot who became sentient, and
KG, a former DPT member summoner who manifested as a horse-fairy to travel the world.
Soujirou is one of the leaders of Akiba, and a harem lead. Former member of DPT and looks up to Shiroe, has his own manga series, and dozens upon dozens of girls. He leads the Nishikaze no Ryodan (The West Wind Brigade)
Nazuna, a spunky Kannagi (Shaman) who loves Soujirou and still follows him in his guild.
That's about it as far as characters who will be in the anime.
KR, Kazuhiko, and Kuina (Intex) likely will not make an appearance in the anime, that's for second season. The rest don't even make appearance in the novels (yet)
Without getting into too much details, yes.
The city of Akiba deteriorates fast and the atmosphere worsen between the residents.
Shiroe
schemes devise a plan to bring 11 of the leaders of the city (including himself) together, to form the Round Table Committee and make them listen to his demands.
Since this is a major spoiler...
No, not yet. Although two characters, including Shiroe, seems to be working on learning the mysteries.
Yes, it's not a harem story. However, Shiroe does get pretty popular with bunch of girls....