
The King of Diamonds is the hardest diamonds game. It is run by an international lawyer and former no.3 in the Beach Keiichi Kuzuryū. It was a game of Keynesian Beauty Contest between 5 people: Keiichi Kuzuryū, Shuntarō Chishiya, Benzō Yashige, Hinako Daimon, and Takashi Asuma. It was completed by Shuntarō Chishiya.
Game Overview
Setup
Player limit: 4
Time limit:
1 minute/round
5 minutes/first round & rounds with new rules
Upon entering, all players shackle themselves to a seat and confirm the operation of the tablet in front of them.
Rules
All players remaining:
 All players select a number between 0 and 100 in the given time.
 The average of the values will be multiplied by 4/5ths (0.8).
 The person who chooses the number closest to the calculated number wins. This constitutes one round.
 All losers will lose a point.
 A new rule is introduced for every player eliminated. On the first round and all following rounds where a new rule is introduced, players are allotted 5 minutes as a way to get used to the rules.
Game Clear: Be the last person remaining.
Game Over: Reach 10 points.
3 players remaining:
 If there are 2 people or more choose the same number, the number they choose becomes invalid, meaning they will lose a point even if the number is closest to 4/5ths the average. The player that didn’t choose the same number wins regardless of their pick.
 If there is a person who chooses the exact correct number, the loser penalty is doubled.
2 players remaining:
 If someone chooses 0, the player who chooses 100 is the winner.
Strategy
All Players Left
Near the beginning of the game, all players assumed that excess logic and mathematics would be the key to the game. However, it was quickly realized by Chishiya that it would be within the facet of rationality to eliminate such abstract ideas such as nash equilibrium, recursive selection, and cognitive limit. Perfect rationality is also perfectly predictable. In truth, the game was simply about reading your opponents and simple calculations. He and Hinako Daimon proved his theory right when they used a seemingly absurd strategy outside all logic— intentionally picking obscure numbers to round the average into a more favorable value— to balance the losses between players so the losses for the King of Diamonds adds up.
3 Players Left
With the addition of the invalidity rule, the strategy for the game completely changes. The amount of practical numbers and tactics reduces. Prior, 0 was a strong number but now there’s the high risk of choosing the same number. But if 0 is avoided and 1 is chosen instead, another player could have the same though process, yielding the person choosing 0 as the winner. If the results are 0 1 2 then the player who chooses 1 wins. But there’s not much difference between 2 or 100 either. In the following scenarios: 0 0 20 0 1002 2 100 2, 100, and 100 win respectively. This was Daimon’s though process. In conclusion, this battle is a three way between 0, 1, and 2100. 2100 may look nice initially, but if the other 2 choose the same the smallest number wins. This elimaintes 3100. This round, all players chose 1. The only winning move would’ve been to choose any value other than 1.
2 Players Left
When there are 2 players left, it is the most ideal for the person who is behind in points to choose 100, and the person in the lead to choose 1 or 0. Although 0 is the most powerful number to choose, it is also balanced by 100.
Synopsis
The winner of each round is in bold.
Round  Chishiya's number  Daimon's number  Ashige's number  Asuma's number  Kuzuryū's number  Winning number 

Round 1  32 (1) 
40 (1) 
30 (1) 
33 (1) 
29 (0) 
32.8x0.8=26.24 
Round 2  17 (2) 
21 (2) 
16 (2) 
15 (2) 
14 (0) 
16.6x0.8=13.28 
Round 3  7 (3) 
11 (3) 
3 (3) 
7 (3) 
5 (0) 
6.6x0.8=5.28 
Round 4  2 (4) 
4 (4) 
0 (4) 
0 (4) 
1 (0) 
1.14x0.8=1.12 
Round 5  100 (5) 
1 (4) 
0 (5) 
0 (5) 
0 (1) 
20.2x0.8=16.16 
Round 6  25 (5) 
100 (5) 
0 (6) 
5 (6) 
17 (2) 
29.2x0.8=23.52 
Round 7  100 (6) 
30 (5) 
0 (7) 
4 (7) 
10 (3) 
28.8x0.8=23.04 
Round 8  20 (6) 
10 (6) 
36 (8) 
34 (8) 
20 (3) 
24x0.8=19.2 
Round 9  6 (7) 
8 (6) 
2 (9) 
20 (9) 
10 (4) 
9.2x0.8=7.36 
Round 10  1 (8) 
7 (7) 
0 (10) ^{†} 
0 (10) ^{†} 
2 (4) 
2x0.8=1.6 
Round 11  1 (9) 
1 (8) 
X  X  1 (5) 
No winner 
Round 12  23 (Exact match) (9) 
62 (10) ^{†} 
X  X  1 (7) 
28.66x0.8=22.93 
Round 13  100 (9) 
X  X  X  0 (8) 
100 overrules 0 
Round 14  100 (9) 
X  X  X  0 (9) 
100 overrules 0 
Round 15  100 (9) 
X  X  X  0 (10) ^{†} 
100 overrules 0 
Trivia
 The Keynesian Beauty Contest was a famous experiment in behavioral economics. People voted from a list of women who they thought was the most beautiful, and the first place women received a prize. A 20th century economist, Keynes, linked these contests to equity investments.
 In summary, it was believed that to earn maximum profit, one mustn’t invest in the market they think is best, they must invest in what everyone else thinks is best. This directly compares to the psychology used in the King of Diamonds game.
 Out of all the face card games, it is likely that due to Kuzuryū’s belief on equality, that this is the most impartial and generous game constructed.
 “If there are 2 people or more choose the same number, the number they choose becomes invalid, meaning they will lose a point even if the number is 4/5ths the average.”— The reason this rule is put into place is to ensure that there can no longer be a nash equilibrium. Because of that, a new sense of danger arises, but there’s also a deeper layer of strategy.
 “If someone chooses 0, the player who chooses 100 is the winner.”— The reason this rule is put into place is to balance the power of 0. By the time there are 2 players left, if anyone chooses 0, they’ll automatically win regardless of the number the other person choose. In that case, the person in lead would continually pick 0. Even if the other player also choose 0, they’ll both lose points but the player with lesser points would die first. To balance that quirk, 100 overrides 0 as the victorious number.
 In addition, by rule 3, the game essentially becomes a game of rock, paper, scissors between the numbers 0, 1, and 100.
 This was the eighth face card game to be completed in the Second Stage.
Gallery
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Clubs [♣]  
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Diamonds [♦]  
Hearts [♥]  
Others [⚂]  Unknown Cards · Unknown Venues 