I played with my 10 yo daughter, and it was a blast, but it can be done with a solo character.
Spoiler (click to reveal)
The books have clear references to a possibility of playing solo, e.g. some books would say something like “if playing with two or more heroes, do X..”, which in my mind implies that one player is another option that is available.
At least play through the Open Roads starting scenario solo... It does a good job of teaching the game and foreshadowing interesting puzzles and mechanics from the game. Before I started I thought that the story would all be in the book, follow the numbers and see what happened. THEN I was made aware of something outside the book I was given to look over. Needless to say, I did NOT give it the attention it deserved and the book called me on it! If I were in a group, we wouldn't have been able to "fix" that issue. Solo, I put the game on pause until I rectified the situation and that opened another whole branch to the game.