Why was Rachel excited about meeting Chaya in The Devil's Arithmetic by Jane Yolen?
The Devil's Arithmetic by Jane Yolen employs a unique kind of flashback experience for the young protagonist, Hannah Stern. She is a twelve-year-old girl who lives with her family in New York and is, shall we say, disinterested in hearing about the things which interest the rest of her family about their Jewish heritage. In fact, Hannah wants nothing to do with their remembrances and reflections on the past, despite the fact that her family--like so many others- was impacted by the atrocities of World War II and the Holocaust.
In a rather surreal moment during a Seder celebration, Hannah goes to the door to see if the prophet Elijah has arrived. When she looks outside, the scenery is nothing like what she should be seeing, and when she looks back inside, she is shocked to see that her house has changed, as well. Hanna soon learns that she has somehow been transported into to Germany in 1942 and her name is now Chaya Abramowicz. She arrived from Lublin two days earlier, following the deaths of her parents. SHe has also been ill with the same sickness that killed them.
On her second day in this vision, Chaya attends Schmuel's wedding with Gitl but feels rather out of place since she does not know anyone. That is when Chaya meets Rachel. Rachel has remarkable green eyes and a welcoming smile. She says:
“So—you are Lublin Chaya.”
Rachel is obviously thrilled to finally meet the girl she has been told about by Gitl, and after joyfully introducing Chaya to her friends, Rachel exclaims that Chaya is going to be her new best friend.
This pronouncement causes Hannah (as Chaya) to finally commit herself to living in this time and place when she realizes that her life in New York would not be acceptable here.
So, the answer to your question is that Rachel has heard all about Chaya from Lublin and is just happy to finally meet her and wants to kind of adopt her as her new best friend.