The girl is in charge of taking care of the animals. Seems like an easy job at first but since you can have 32 animals she's got a lot of work to do!
Besides just taking care of the animals, you have to make sure your boy partner does a good job. If you're nice to your partner you might end up getting married; once you get married though it's Game Over for you.
She doesn't do too good of a job starting out, but she does eventually get better. Maybe one day the two of you will get married and have children!