Well, as far as I know it, anyway. I've never read the comics, although I have seen some episodes of the animated tv-series.

For one thing: it's not Tintin, so don't bother trying to find Captain Haddock, Professor Calculus or any of the other characters. You don't need to watch the 2011 movie, either, other than that it's apparently a nice movie.

The main characters are Spirou and Fantasio (obviously), who are journalists. If my memory serves me, Spirou is the real hero, with Fantasio being more of a comical sidekick. Together they get into strange adventures (mushrooms, memory potions, and intruders sound about right) usually across the border in some far-away land.
They have a pet squirrel (in this maffia game obviously played by Larry) called Spip. It doesn't talk in the series (although his thoughts are used as comic relief) and is a helping hand on a number of occasions.
There's also an occasional character the Marsupilami, an black-spotted yellow monkey-like animal that Spirou and Fantasio found in the rain forest. They have an incredibly long, strong, flexible and prehensile tail which can be used for almost anything. They can be recognized by the sound "houba" and the fact that the terrorize everyone by bouncing around and bumping into things. (Anybody remember the song "Hooba hooba hooba hop hop hop"? It could've been a Dutch band who did this for carnival years ago. Well, anyway, that's the one.)

The Count of Champignac is an old aristocrat with his own castle. He's a friend of both S&F. He's an inventor (whose inventions usually start of the adventure - as this seems to be the case) and a scientist who works (in true comic book fashion) in many fields of the natural sciences at the same time. And obviously (also in comic book form) he's a genius.

There are several other recurring characters, most of which you can probably find on the Wikipage (see below). The page also lists enemies, and when you click on any of the characters' names, you get different page with a list of characters. For a Maffia game (which may depend on misdirection, misinformation, hints, red herrings, etc), I'd say you can pay special attention to these recurring enemies:
- Zantafio, who bears a strong physical resemblance to Fantasio, because they are cousins;
- John Helena "la Murène/ The Moray", a criminal (but reformed when last seen) when adventures take place in the water (therefore unlikely if we're cooped up in the castle, but worth listing anyway);
- Don Vito "Lucky" Cortizone, New York mafia boss (same here, although he could have sent henchman to do steal one of the Count's inventions);
- Cyanure, a robot in human shape (if I remember correctly, unable to distinguish from humans unless you know her face).

Any additions are appreciated.