Jack of Hearts (Solitary Confinement)

Additional Players: Sunato Banda, Oki Yaba

Citizens: Enji Matsushita - Jack of Hearts

Player Limit: 20

Time Limit: 1 hour per round

Rules

Players must equip an explosive collar upon entry. Before the game begins, a mark (of a card suite) appears behind each player's collar, renditioning everyone unable to see their own mark (Spades, Diamonds, Clubs or Hearts)
Players are not allowed to look at their mark using any reflective surfaces or materials
Players are however free to talk amongst themselves during each round
In the final 5 minutes, all players must enter a cell and announce the mark that they think is on the back of their collar. If they're correct, they move onto the next round. Their mark changes, and the cycle repeats (a new round starts). If the player remains silent or says the wrong mark, it is GAME OVER (their collar explodes)
Among the players is the Jack of Hearts. When the Jack of Hearts dies, the remaining players all survive and it is GAME CLEAR
If only two players remain (Jack of Hearts included), only the Jack of Hearts will live, since their identity will become apparent

Details

In the prison, there are no reflective surfaces and it is so large and spacious, it is subtly encouraged for players to have discrete conversations discussing strategies. Half a year's supply of food was provided, suggesting the game is a game of attrition.

Solution

The Jack of Hearts' identity was deduced by both Sunato Banda and Oki Yaba after they formed a sudden but strong alliance and trusted each other to see that the only other remaining player was the Jack of Hearts.