Worn, it makes one invisible to physical beings and also extends the life of the possessor. Not just that, but it also enhances their perceptions, allowing them to see things that might be hidden from others (for example, Frodo was able to see Nenya on Galadriel's finger when Samwise could not).
The One Ring carries a fragment of the Dark Lord's malice and desire to dominate all life, allowing it to influence actions as well as corrupt the possessors. Philosophically, it can be taken as a more modern version of Plato's Ring, or a symbol of the immoral life. The Ring promises power to various characters in various degrees, depending on their perceived significance (to the Ring) to the fate of Middle-earth. Galadriel, for example, would have become a powerful queen whom all "shall love... and despair," whereas for Bilbo and Gollum it chiefly conferred invisibility (and longevity). So in actuality, those characters that receive lesser powers from the Ring are, ironically enough, more vital to its destruction.
Some Mary Sues have created extra Rings of Power. Other Mary Sues claim to be immune to the Ring's power, which, as evidenced through Galadriel and Sam, would take an immense awareness of self and personal limitations and many Mary Sues are simply incapable of that. It is not unknown for a Sue (most often replacing or lusting after Frodo) to bear the Ring. Naturally, this is a serious charge.
Occasionally, PPC agents have had to handle the One Ring, usually because a Mary Sue has replaced Frodo as the Ringbearer. Thankfully, the canon usually shields them from the worst of the Ring's effects, but being so near the Ring is still not a pleasant experience.
Known Bearers of the One Ring
This list only includes those known to have worn it.
- Sauron (the one who forged it)
- Bilbo Baggins
- Frodo Baggins
- Samwise Gamgee
- Tom Bombadil (who wore it momentarily, though he did not really "bear" it anywhere)
- Most definitely not Mary Sue.