If my memory is accurate, there have been six champions in the WrestleMania era to win a championship at a WrestleMania, hold the title for an entire year, and then successfully defend their championship at the following WrestleMania:
Demolition, WrestleMania 4 (WWF Tag Team Championship)
Brock Lesnar, WrestleMania 33 (WWE Universal Championship)
Becky Lynch, WrestleMania 35 (Raw Women's Championship)
Roman Reigns, WrestleMania 38 (WWE Universal Championship)
Bianca Belair, WrestleMania 38 (Raw Women's Championship)
Rhea Ripley, WrestleMania 39 (SmackDown Women's Championship/Women's World Championship)
Cody Rhodes joins Randy Savage (WrestleMania 4) and Hulk Hogan (WrestleMania 5) as the only champions TTBOMR to win a championship at Mania, hold it for an entire year, and then fail to defend their championship at the following Mania.