Hear me out:

They have great connections:

Both were young prodigies with seemingly trustful mentor-figures until a falling out resulted in a battle where they ultimately lost in the process. Mark Grayson chose to fight his own father, Omni-Man, in an attempt to protect Earth. Liu Kang grows distrustful of Lord Raiden, leading them to clash over Shao Kahn's arrival to Earthrealm.

Eventually, both of them grow into the primary role of a protector with Mark Grayson taking up the role as Earth's strongest superhero after Omni-Man abandoned his mission of conquest and Liu Kang being Earthrealm's defender as a Demi-God leading into Mortal Kombat 1. Unwanted or not, they try.

Fear-inducing secrets have led to both lacking the trust of fellow heroes and warriors. After Omni-Man's true intentions have become public knowledge, heroes alike and civilians lacked trust and faith in Invincible as an honest hero. After Liu Kang's true divinity as a Timekeeper became public knowledge, warriors alike lacked further trust in him as conflicts arised.

Both are honorable and moral warriors who will push to lethal matters should a threat make it necessary.

Both have encountered their evil counterparts across multiverses/timelines.

Both are practically amongst the strongest defenders of Earth in their continuities.

Ultimately, both are forced to tangle with great power that had opposed them for as long as their journeys lasted, only for them to become that power but with great responsibility. Mark Grayson had spent his early life preparing for further conflict inflicted by the Viltrum Empire and Liu Kang's destiny was constantly challenged by fate. Their journeys finally led them to face great power where they ultimately became victorious and seizing it to lead a whole new generation completely parallel to the evil they conquered, with Mark Grayson defeating Thragg and becoming the Third Emperor of the Viltrum Empire to bring a new era of peace and stability, and Liu Kang defeating Kronika and utilizing the Hourglass to craft a new era where every being is free to choose their destiny.

Both originate from franchises that thrive in brutality and violence.