Most IWRG trainees work a handful of FILL lucha libre shows before either graduating to the main roster or leaving the FILL program. Blue Monsther is the exception, having worked trainee shows and only trainee shows since 2011. He's never made an appearance on the main show (and so is technically not an "IWRG" wrestler but charitably listed in that category for simplicity), he's rarely seen on other shows, and he's only once been involved in the main event school vs school cibernetico - and even that only happened because it was FILL vs FILL intraschool battle.
Either Blue Monster is the kid who just can't graduate or he's secretly a more experienced luchador using a trainee gimmick to help out others. (He seems heavier than Bombero Infernal, who would be the most likely person for that sort of role.) Weirder still, Blue Monster has also done MMA.