In this we have a case of

1) There is a clear demarcation between Good Guys and Bad Guys. Or, more to the point, it is made expressly clear that Fantasio is a Good Guy (don't know about other characters). He's not tinged with darkness, he doesn't have subversive tendencies. So, barring outsider influence, he's a good guy.

2) There's no flavor justification for Fantasio suddenly being a bad guy. Maybe a mind-control ray or something, but that's something the inventor would create, and he's friends with Fantasio.


2) Remember, we're not trying to find evil aliens or whatever, we're trying to find TRESPASSERS. Fantasio is expressly a friend of our host, the inventor. So even if I had been zapped with an Evil Opposite Ray or whatever, I'd still be invited