St. Bernard Split Personality

Hello proud dog owners.

This is my first post with pics of the wolf in sheep's clothing to come later. Kidding, she's actually pretty awesome most of the time.

We have recently become parents to a St. Bernard puppy and have had her for about a month. She's been up and down through potty training and overall has been very good. When I'm around (male) she is very obedient without any sassiness or testiness, but when I'm not, she gives my girlfriend and her son a hard time. Some examples are defying any command, biting or nibbling on ankles and clothes, digging outside, jumping on couches and coffee tables, etc. My girlfriend is an incident away from calling a priest to perform an exorcism. We don't undertand what her deal is when I'm around as opposed to when I'm gone. She scolds and shows the same demeanor of disciplining that I do, but her efforts lack the same results as mine. Does anyone have any suggestions for this Jekyll & Hyde behavior??

Thanks!