In single player, you create the one main character, and can recruit companions afterwards (with up to 3 in the party at a time). In a multiplayer game, each player creates one character, and if there are less than 4 players, the remaining slots can be filled with companions later.
When companions are recruited, you can choose their starting preset (it isn't full customization, but a lot more flexible than D:OS).
For dialogues with the companions, they state their opinion and you choose what action to take. Their relationship with the main character is affected accordingly, possibly even resulting in them leaving the party.
It is possible to open multiple instances of the game to join a LAN game on the same computer (depending on system specs, dropping the resolution and switching to windowed mode first will make things smoother), and then you can create up to 4 custom characters, and once in-game, save, exit the client instances and continue with the custom party.