This is the 8048th most frequent Russian word.
кристиной
"Кристиной" is the instrumental case of the name "Кристина," meaning "by/with Kristina" in English.
In this sentence, 'Кристиной' is the instrumental case of the proper noun 'Кристина', used after the preposition 'с' (with) indicating companionship.
Я познакомился с Кристиной в парке.
I met Kristina in the park.
Here, 'Кристиной' is used in the instrumental case post the preposition 'с' to indicate the recipient of the action.
Мой друг всегда делится своими историями с Кристиной.
My friend always shares his stories with Kristina.
In this case, 'Кристиной' in the instrumental form denotes a collaborative or accompanying manner.