Which character had the greatest impact on Maniac in Maniac Magee?

The character that has the greatest impact on Maniac is Amanda Beale, because she gives him a family.

Maniac wants a family more than anything else. His parents are dead, and the relatives he was sent to live with did not seem able to handle having a child, or each other. He ran away and ended up in Two Mills. There he eventually found Amanda Beale, and they became friends.

The author tells us that...

The character that has the greatest impact on Maniac is Amanda Beale, because she gives him a family.

Maniac wants a family more than anything else. His parents are dead, and the relatives he was sent to live with did not seem able to handle having a child, or each other. He ran away and ended up in Two Mills. There he eventually found Amanda Beale, and they became friends.

The author tells us that Amanda was the first person to befriend Maniac.

As for the first person to actually stop and talk with Maniac, that would be Amanda Beale. And it happened because of a mistake. (Ch. 3) 

Maniac tells her “hi” and she is surprised, because white kids do not greet black kids. He shouldn’t even be in her neighborhood. However, Maniac has a disarming smile and is enamored of Amanda’s books. Amanda carries her library with her, so that younger siblings do not destroy the books. 

Maniac asks Amanda to lend him a book. She is hesitant at first, because she doesn’t know how he will get it back to her. He told her he was from Bridgeport. She is late for school, so she lends him a book about the Children’s Crusade. Maniac loves reading, and he is thrilled.

The Beales take Maniac in and give him a family, despite the color of his skin. It makes little difference to them once he is in their house. Maniac is so excited that he can’t sleep on his first night there until he looks at the house's address.

Before the puzzled faces of Mr. and Mrs. Beale, he opened the front door and looked at the three cast-iron digits nailed to the door frame: seven two eight. He kept staring at them, smiling. Then he closed the door, said a cheerful "Good-night," and went back to bed. (Ch. 12)

Since running away from home, Maniac has wanted nothing more than an address, with real numbers. The Beales give him that. He is forced to leave because others do not approve of a white boy living with a black family, but he later returns after Grayson dies, finally home.