Cartoon Network vs Honkai Impact 3rd

Connection:

Both are teenage protagonists who were orphaned after a catastrophic event (the Nanite Event and Second Eruption respectively) that caused most humans to mutate (Nanites turned people into EVOs, and Honkai energy turned people into Honkai Zombies) and killed their parents, forcing them to take residence with a violent faction (Quarry's gang and the Siberian army respectively) and serve as a top agent in their ranks (Rex was one of Quarry's lieutenants, whereas Bronya became the Silver Wolf of the Urals thanks to her incredible skill as a sniper)

Both are known for their massive affinities in the field of technology and were eventually taken in by a manipulative individual that would later try to control them (Van Kleiss and Cocolia respectively), only to later be recruited into an organization that fights against the monsters that plague their world by one of its members (Providence's top agent, Agent Six, brought Rex in to Providence, while Bronya was recruited by Himeko to become a Valkyrie of St. Freya Academy), where they quickly gained notoriety because of their extraordinary attributes (Rex's ability to cure EVOs, and Bronya's high-level Honkai adaptability)

Both had also previously took part in an experiment meant to enhance human beings (the Nanite Project and X-10 Project respectively) that, due to an accident, made one of their loved ones disappear for some time (Caesar Salazar and Seele Vollerei, respectively), though for contrasting reasons (Rex was injected with Nanites so that his life could be saved after he was injured in an industrial accident, while Bronya threatened to commit suicide if the procedure wasn't applied to her). This experiment, however, caused both to gain mental problems (Rex became an amnesiac, while Bronya was unable to feel emotions or walk due to her prolonged exposure to Honkai energy), but gave them access to a piece of equipment that would later become their main method of attack and morph into just about any military weapon or tool available to them (Nanites and Remodelled Bunny 19C respectively)

Both also obtained an important item that's closely connected to the monsters that roam their world (the Omega-1 Nanite and Core of Reason respectively) and gave them access to a powerful and nigh-unstoppable form (the Omega-EVO form and the Herrscher of Reason respectively)