Pretty much any native Spanish speaker aged 10-30. It's as dominant now as Timbaland/NERD were in the UK/US in the 2000s.
People need to stop using the word underground in relation to anything in Ibiza, because it just looks a bit ridiculous. The free party psy scene was the last underground in Ibiza and they killed it. Perhaps DC10 in the very early years too before they sold out. In other countries, I see things like Dekmantel which obv buy into an underground ethos and host some excellent DJs (not music you'll ever hear on the high street) but nevertheless it's still incredibly safe, tame and middle class, light years away from the real underground that defined warehouse culture in dangerous parts of town where you had to climb through some fence and get past a psychopath 'security team' run by Toothpick Terry with a swan on his neck and hope the cables didn't run through pools of water and where on the dancefloor you would genuinely find yourself surrounded by students, gangsters, crusties and even pitbulls all in the same room whilst some projector blasted mad lasers and the DJ played records that blew your senses apart.
Much like balearic, underground was never a type of music but a way of life, for the rebels, misfits and outcasts or in the case of 80s kids in East Berlin an act of political defiance against the stasi. I'm not gonna pretend to be Mr Underground because I'm too young and only really caught the tail end of the London squat scene late 90s/early 2000s, but once you've caught a taste of this parallel world, going to a normal nightclub is never the same again. So bringing it back to Ibiza, sure you can still have a lot of fun and places like Pikes and Akasha are in it for some of the right reasons but let's not pretend any of this is edgy or subversive in any way. So those much coveted after parties that FGP and his 'bros' (real or imagined) are so desperate to gain access to are simply a status symbol, because they're seduced not by the quality of the event but the secrecy lending them the kudos. Turning up at someone's villa to hear them playing anodyne tech house dross is not underground, trust me...