Most WrestleMania losses: Triple H - 13

Unfortunately for Triple H, he does hold a dubious record that's unlikely to be broken - 13 WrestleMania losses.
At first, that wouldn't appear to be the case. Triple H has been the subject of infamous WrestleMania controversies and "burials" over the years, most infamously involving Booker T at WrestleMania 19. Triple H was also the first heel to walk out of WrestleMania as champion, as seen in 2000, and has even scored a WrestleMania victory over the 'beast incarnate' Brock Lesnar.
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But surprisingly, despite these victories, Triple H has lost more times than anyone else on the Show Of Shows, walking out of WrestleMania 12, 15, 17, 20, 21, 22, 24, 27, 28, 30, 32, 33, and 34 as the loser.
This is an unlikely record to be surpassed, especially if Triple H keeps adding to it, which seems like a probable scenario, even if not this year. Many superstars would be lucky to even appear at 13 WrestleManias.