As part of the Tarokka card reading, this is one of the possible allies (under Strahd's Enemy, p. 17):
A. Tempter (Queen of Diamonds)
I see a child-a Vistana. You must hurry, for her fate hangs in the balance. Find her at the lake!
This card refers to Arabelle (see chapter 2, area L). She gladly joins the party. But if she returns to her camp (chapter 5, area N9); her father, Luvash, refuses to let her leave.
However, both of the other sections this references, area L and area N9, do not mention anything about how this character could be useful to the party, given that:
she's a seven-year-old girl with no stat block to imply that she's a combatant, not that I'd expect that of this character, or that she has any powers or knowledge or anything.
This character isn't stated to be able to help the party at all against Strahd. Is this something for the DM to come up with by themselves, or am I missing something?