I think one big reason why a lot of us here are so invested in Geno and why his Smash demand spans nearly two decades is because of the ending of Super Mario RPG itself. The game didn't just end, Geno leaves his body in a very "my work here is done" way after the team defeats Smithy that made it very apparent to even little kids that Geno was probably never coming back. Up until that point and arguably still, SMRPG had the most emotional ending of any Mario game and didn't have that sort of "see you next time" vibe that a vast majority of Mario games have. SMRPG was kind of like closing a good book that you wish didn't have an end but you were still happy for the characters because, as its ending showed, their wishes came true and their story arcs (at least for the original characters) were complete. In a way, Geno represents what was a major breakout title for the Mario franchise that took a lot of risks that played out in spades and set new standards for what the Mario series could do not only with music, gameplay, and presentation but also story and the personalities of its characters.