Technically, every player in the game is the supporter, and every player is not. Because the concept that a player may be officially designated the supporter would create a situation of impunity (who watches the watcher?), and players are told that even the supporter may not know that is there role, it makes much more sense that the players all can fulfill this purpose.
There is a point at which one player will look at the actions of another and feel they are self-serving, and decide they need to put an end to them. They may try to confront the other party, use their funds to hinder or redirect them, or destroy their efforts. But in the end, if they come to believe they have a moral out in the role of the supporter, and virtually unlimited funds to get them out of the legal trouble, the societal justice impulse could kick in, and they may kill the player.
There is also a possibility, based on the nature of Juiz, that Juiz may also help fulfill this role. There may be certain conditions or thresholds that will trigger actions by the long arm of the concierge to take actions against misguided players. This is not evidenced (that I can remember), but it is also possible that Juiz may indirectly responsible for a player acting in the role of the supporter, by ensuring that actions that would trigger them to retaliate will come to their attention.
Alternatively, self-serving players may be fed information that will cause them to act offensively to another, instigating retaliation from the offended by the information they, in turn, are delivered.
Also, keep in mind that a player overtly acting as the supporter would be seen as a threat to every other player, and would almost certainly become a target themselves, since transactional data becomes public upon use.
All theoretical, but there may be something there. Pleasure to serve. Nobelesse oblige.