This is the 3716th most frequent Polish word.
krystiana
"Krystiana" is the Polish feminine form of the name Christian, meaning "follower of Christ."
Here, 'Krystiana' is used in the accusative case to refer to the direct object of the sentence, indicating who was seen - the proper noun 'Krystian'.
Widziałem Krystiana wczoraj w parku.
I saw Krystian in the park yesterday.
In this sentence, 'Krystiana' is in the genitive case, indicating possession. It denotes that the book belongs to Krystian.
To jest książka Krystiana.
This is Krystian's book.
In this example, 'Krystianem' is in the instrumental case, used here as part of a prepositional phrase to describe with whom the conversation took place.