Saw this thread and decided to give it a shot...
> Engagement Ring
#550 (every subsequent purchase increases the price)
Requirements: Must have Endosomatic Heart purchased
The species of the last befriended ghost will heal you when they eat you.
However, the item will be lost if the ghost is flashed.
There is also a 2% chance of triggering an instant gameover (with a special cutscene) when you're eaten by the befriended ghost.