Well, we only see Rei's grandfather in the anime, who she calls "ojii-san" (Japanese for grandfather.) Her father and mother are revealed in the manga. There's a detailed special on that and you can check it out by clicking here!

How is Rei different in the manga and anime?

I have discussed this extensively in a section called In Black and White, to know about her in the manga. The manga Rei is really different from the anime, and she's not as close to Usagi in the manga.
But the basic difference is that in the anime, Rei is more vocal and more funny, and in the manga, she's more reserved, quiet and blunt. Her beauty and that she is a very religious Shinto priestess is more emphasized in the manga.

What is that sign on Sailormars's forehead?

Oh, if you see the lovely picture of Rei above, that sign on her forehead is actually the planetary symbol of Mars. You see that sign on all the senshi's pens, too. By the time she was revealed as the princess of Mars in the manga (and so are the other senshi), they'll be bearing that symbol on their forehead, like Serenity bears the Moon symbol on her forehead as Moon Princess. As Sailorstarmars, she'll also be having that on her forehead instead of a tiara.

Why can't the villains attack while the sailor senshi are transforming?

Okayyy... I honestly thought it was silly at first, but well.... Anyway, what we see in the anime are somewhat "enhancements" to add more interesting things! They are interesting, aren't they? In the manga, there are no transformation sequences as that in the anime. These transformations actually exist instantaneously, meaning that once they call on the transformation, they change right away!

Take for example, Chibiusa's in S, where she didn't have a henshin ("transform") sequence, but changed into Sailorchibimoon when she called on her henshin phrase. And in S, we saw how Sailoruranus returned back to Haruka in only a matter of seconds. My guess is that these transformation sequences exist in suspended time, something like an inbetweener in time sequences that makes the sequence suspend from real time. But mostly, they're there to add more interesting stuff in the anime!

What is written on Sailormars's ofuda?

The script used in her ofuda is kanji, Chinese script used in Japanese writing. It is translated simply, "Evil Spirit, begone!" Rei uses this along with her chanting to ward off evil spirits or usually to halt the enemy in their tracks. This ability of hers is very useful indeed, taken from her training in the Shinto shrine.

Is Yuuichirou a street beggar?

Actually, he isn't. I really don't know where this came from ^^; but he really is a rich guy. He owns a ski lodge, which is a big mansion, and Rei wondered why a rich guy like him wants to work in Hikawa Jinja. The first time Yuuichirou saw Rei, he is said to have fallen inlove instantly with her and immediately her grandfather that he may work in the shrine.