Mine is similar. She'll hide if someone comes in but if they stay awhile she may eventually pop out and check them out at a distance especially if it's a female voice.
It took six weeks to integrate them but worth it in the end :)
Mine hates other cats, dogs and all people apart from me and my lady. Literally runs and hides in the gap in middle of our pillows even at the voice of ppl.... (he was a rescue kitten, nothing we’ve done)
He has been my biggest weakness for the past 14 years - I can't resist that belly! Aww! Your moggie on the right looks like he/she is thinking "Did someone say Dreamies???" Oh no! At least she has more love to give to you! :3
It's great when cats have a fellow cat friend. Mine hates all other cats. :(