Keeper - Tradução em português

Hey guys, what's up? I didn't quite the meaning of the word 'keeper' in the following context:

Kristin:Hey, I just got an e-mail from Katy.
Joe:Oh, did she make it home okay?
Kristin:Yeah, she did. It was a really nice e-mail. Shewas thanking us for the nine
days that weput her upwhen she was here and also for the great time thatwe
showed her.
Joe:Yeah, I mean, thinking back, we really crammed in a lot.
Kristin:We did. She also said she enjoyed meeting you and getting to know you.
She, uh, confided in me that you’re a keeper.

I've been told that it means 'someone good to continue dating', but how so????
It seems to make no sense to me, how would it be in Portuguese?