#1 The move set and in-ring behaviour
Brock Lesnar is a huge guy who moves like a cat. His agility is tremendous, and quickness is unbelievable. Even though he resorts to only using suplexes and F5s during his matches now, there was a time when the Beast Incarnate did top rope moves.
Lesnar's finisher during his days with Ohio Valley Wrestling (OVW) included a shooting star press. Even though he messed up a shooting star press at WrestleMania on Kurt Angle, the world was shocked at Lesnar's athleticism.
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Sullivan is also quite similar inside the ring. Like Lesnar, Sullivan also prefers power moves that dominate his opponents. The Freak has several variations of bodyslam. However, when the time comes, Sullivan also has no problem ascending the top rope and doing a diving headbutt or an elbow.
Individually, the points mentioned above do not imply Lesnar and Sullivan are identical, but on the whole, there is an uncanny similarity between the two. The claim is even more solidified when we remember that WWE planned Sullivan to take on John Cena at WrestleMania 35.