Never played this. Someone just told me that the game is great fun as solo experience. Is that true? I'm not too much into MMOs, but if I can build a party (he told me about those "hero companions") and explore a world with cool places to find then I'm in 100%
He praised them for that ... just curious how do I get them? Early game? Late game?
There are two types of NPCs you can add to your party: henchmen and heroes.
The henchmen are what is available in the Prophecies (original game) and Factions (first expansion) campaigns. They are quite weak, their skill bar is suboptimal and they are not customizable. The point was that ArenaNet wanted you to party up with other players to complete missions, and maybe add 1 or 2 henchmen to fill up the party if nobody else was available. Still, both Prophecies and Factions can be finished with henchmen only, but you might not be able to do the bonus objectives during the missions, or the most difficult quests.
The heroes were introduced in Nightfall (second expansion), and these are fully customizable NPCs that you unlock throughout that campaign. You can change skills, attributes, equipment, they have the same customization options as your main characters. The AI is quite good, some build/heroes combinations can tear through the game by themselves. There is a progression system where you need to unlock skills for these heroes.
The problem is that to unlock heroes you have to play Nightfall first (you can travel there from Lion's Arch if you start with Prophecies), which I don't think it's optimal, the story goes: Prophecies -> Factions -> Nightfall -> Eye of the North. I'm not sure if there is a guide in which you can rush through the first part of Nightfall to get the heroes and then go back to Prophecies.
So yes, the game can be played fully in single player, with caveats. With this update people are coming back to play the game, so it would be worth to party up with real people to do the missions, it's much more fun that way. There is a party search system, you don't need to socialize if don't want to. You can join a party, complete the mission and leave.