You can solo and the main story assumes a central "main char", but you get to create a full party of up to 6 characters. Of course, you can decide to solo the game, but it's not recommended unless you know how to cheese the system

To address S-man's points one by one:
1: creating more than one adventure is a bit of a chore.
You consider it a chore, others consider it great fun. We promised full party creation in the Kickstarter and we will deliver it.
2: a well made rpg can have followers which can be balanced to reflect the game.
Followers like in Baldur's Gate with their own companion quests, romances etc take a lot of work to create. We would rather use the time and effort to create more sidequests that can be enjoyed by all players, without the need to recruit a companion.
3: By having a hero with followers having to build a balanced party with a limited number of followers, you are trying to make due with what you find as the journey goes on which is something I think was great about the bg series (you start with no one and over time assemble a working group to solve the problems).
By creating a full party from scratch, the player can tailor the characters to his liking. He can also create his own challenge runs, for example a full 6 wizards playthrough or a 6 fighters playthrough, which you couldn't attempt in a game without full party creation.
4: party interaction and trying to keep all party members with different outlooks etc.
5: Followers can be tied to subplots to make the game more interesting, and provide plot hooks. Given the SIZE OF RB, that would be cool.
Again, it's a matter of effort and logistics. We only have so many people working on the game, and so much money to finance it. Do we invest our time and resources into companions, or do we invest them in more sidequests?
Player-created party members have backgrounds in RB, so they sometimes offer their opinion on things even if they're not the active talker in a conversation. A bit like Wizardry 8 where your characters would often comment on things.