Currently it is in the possession of Kaito, who at first refused to bond with it. But after a while, due to the dangers that the Sage Puzzles and realising his abilities alone would not be enough, he reconciled to use it.
When Kaito's brain passes a certain stress threshold, the Armband of Orpheus activates to protect and assist its wearer. It begins to glow a bright golden light, followed by Kaito's character change, and his right eye turning red.
The armband of Orpheus stimulates the brain, allowing it's wearer to use their dormant abilities. However, Kaito's circumstance is more complicated. When his brain is being stimulated up to 100%, it even uses areas of the brain meant for emotion to solve a puzzle. This is why his character would change instantly, switching to being more reserved and focused and thus allowing him to concentrate even in the most stressful situations.
However, should the wearer's brain become controlled entirely due to emotions, it can cause what Cubic called Reverse Rampage. In Kato's case, because of how his parents died, he had at one point become afraid of puzzles. Remembering what had happened to his parents when Nanoha and Gammon had nearly died trying to solve a puzzle when they were invited to meet Rook, he had gone into a panic attack.That had activated the Reverse Rampage. His brain had been set on the fear of puzzles and due to him passing the limit stress threshold his armband activates. The brain, controlled by the memories, senses all puzzles as danger and Kaito rejected them. The armband being unable to connect to the brain, causes artificial shocks through his system that are dangerous to his health.
There are also other types of Orpheus jewelry seemingly invented by the Order of Orpheus whose members all carry Armbands of Orpheus. These jewelries include earrings and rings.