For me, Kira agreed to take MC to the island. She changed her mind without any prompting from her father (who knew nothing about it), or anyone else. She made the decision on her own to betray him to try and earn her father's respect/trust/whatever. She betrayed MC and broke an agreement all on her own. (I'm actually a little annoyed that we are even forced to like her and drool over her, I never want to see or talk to her again).
Pearl didn't make any agreement with MC to not take the relic. So she didn't betray him. She was also coerced into doing it or their father would kill Kira, she really had no choice, yet stopped at the end and helped MC anyway. She didn't do anything except steal something from him.
While I'm forced to drool over Kira, I'm getting my revenge by adding Kelly & Pearl to my harem - and not her! (I'm petty like that.
)