She is a brahmin woman married to the rishi Devasharma. Her husband hears that Indra was in the vicinity and immediately remembers Ahalya and decides that Ruchi will also cheat on him with Indra.
Then, Devasharma has to leave the ashram for a few days, and so he asks his student Vipula to keep an eye on her when he is away. Devasharma, before leaving, reminds Vipula that Indra can take the form of any human or animal to try to sleep with Ruchi.
Once Devasharma leaves, Vipula decides that the best way, according to him, to keep Indra from seducing Ruchi is to possess her body with his tapasya powers, and later escape via her menstrual blood.
So, Vipula goes and distracts Ruchi by sweet-talking her and then 'possesses' her by first locking eyes [honestly though the description sounds more like he tricked her into nasally/orally consuming some kind of paralytic]. He then touches his limbs to hers, his lips to hers, and his privates to hers to complete this 'protection ritual'.
Whatever the nature of the 'magic' might have been, when Indra does come to the ashram as a guest, Ruchi, despite being completely awake and aware, and wanting to be a good hostess to him, is unable to control her limbs or facial muscles.
Seeing her unable to respond, Indra loses interest in his plan, and then Vipula shows him out of the ashram.
Later, the universe decides that Vipula's actions with Ruchi could be termed as assault, and so various people inform Vipula that he's going to hell. Devasharma, however, uses his tapasya powers to dissolve this sin of Vipula's, and he eventually goes to heaven.
