Is a person alone in the bathroom, and can jinn be present with him in it, and if so, what steps should be taken to preserve modesty?

The jinn see humans, but humans do not see them. Since devils frequent impure places, such as restrooms, they are present there to harm humans. To protect oneself from the evil of devils upon entering the restroom, a Muslim should say: "Bismillah, Allahumma inni a'udhu bika min al-khubuth wa al-khaba'ith" (In the name of Allah, O Allah, I seek refuge in You from male and female evil spirits). Saying "Bismillah" acts as a veil between the eyes of the jinn and the private parts of the children of Adam, and "Allahumma inni a'udhu bika min al-khubuth wa al-khaba'ith" is seeking refuge in Allah from evil and its perpetrators, meaning the devils.