The “Adored Unknown” in The Adventures of Tom Sawyer is a girl.  She is a girl that Tom first sees in Chapter 3 and with whom he immediately falls in love.

At the point that Tom sees the girl in Chapter 3, he has been in love with a girl named Amy Lawrence.  But when he sees this new girl in town, he forgets all about Amy and he falls for this girl.  We see this happen in the following lines:

he saw a new girl in the garden--a lovely little blue-eyed creature with yellow hair plaited into two long-tails, white summer frock and embroidered pantalettes. … A certain Amy Lawrence vanished out of his heart and left not even a memory of herself behind.

Later in the chapter, she is referred to as “the Adored Unknown.” 

When we get to Chapter 6, we learn the girl’s name.  Tom comes to school late and sees her sitting alone at a bench on the girls’ side of the room.  He intentionally gets himself whipped.  He knows that, after he is whipped, the school teacher will further humiliate him by making him sit with the girls.  Since the Adored Unknown is the only one with an empty space next to her, he knows he will get to sit with her.  His plan works and the following exchange ensues:

What's your name?"
"Becky Thatcher. What's yours? Oh, I know. It's Thomas Sawyer."
"That's the name they lick me by. I'm Tom when I'm good. You call me Tom, will you?"

From this, we know that the Adored Unknown is a girl named Becky Thatcher.  She will continue to be important throughout the book.