I can see how he was always there or there abouts, but Sin was always seen as the "big bad guy" in the game and I had no emotional attachment towards Yu-Yevon at all when I fought him. On par with Ultimecia, really, who was always technically in control of the other sorceresses to some degree but in reality we didn't get to see her personality until the end. Neither as bad as Orphan, though, who had absolutely no personality whatsoever.