Liza is a witch who’s buried outside of the graveyard in “unconsecrated ground,” also known as Potter’s Field. The main graveyard is built on land blessed/consecrated by a church.
Bod met Liza after climbing a tree branch to get an apple, which soon broke, and he landed and sprained his ankle. Enter Liza Hempstock, a girl who was drowned and burned to death because the townsfolk accused her of witchcraft. They drowned her to see if she was a witch, because if she wasn't one, she would drown. Since Liza was in fact a witch, she survived the ordeal, only to be burnt at stake.
Bod and Liza begin to learn about each other, and Bod tries to help her, and Liza assists him. One thing Bod wanted to help Liza out with was getting a tombstone to mark where she lies, because she helped him he when he sprained his ankle. To do so he attempted to sell a brooch buried with the master. However, the greedy salesman who he brought the brooch too locked him in a room and planned to keep the riches for himself. Until, of course, Liza came to help Bod get out. Liza has also alerted Silas when Bod got arrested, and she helped him when he was battling the Jacks. Bod did thankfully end up getting Liza a headstone, a lovely glass paper weight with her initials painted on.
All in all, Liza became a very helpful friend to Bod, and he in turn, was a friend to her.