Guests, after all, are used to either help attract a wider audience to a game, or to get a wider audience interested in characters by making them appear in a major franchise.

In my case, while he wasn't exactly a guest, Captain Falcon's appearance in Super Smash Bros did get me interested in the F-Zero series. I doubt it's a concidence the series was revived afterwards.

Also, I didn't have that much interest in the Soul Calibur series before the guests in SC2. Eventually, I grew more interested in the series, and now appreciate some of it's characters just as much as some of the guests.

I didn't know Cole Macgrath, the Infamous series nor Toro and Kuro all that well before their appearance in Street Fighter X Tekken.

These are just a few examples. What about you?