Two repetitive foes of Jaguar and E-Man first appeared as alien exiles turned invaders, each were "heady" villains that used superior mental abilities and technology from their worlds to subjugate Earth. Alas for both, they had their hands full with this pair and their allies. Still, the persisted in their dauntless schemes to rule their adopted home.
Brain Emperor's initial (and sole silver age), appearance was Mighty Comics #1. A mental mutant from the planet K-Shazor, his brief reign of terror was squashed by a solar eclipse. His stellar travels led him to a blue world, third from the sun Sol. He once more attempted to cause an entire global population to bend to his will, but soon found that his own device backfired when used by his new adversaries, the Mighty Crusaders. Using their combined willpower, they banish this baddy.
He returned years later, coinciding with the Crusaders who themselves left behind retirement for their spandex superheroics. Realizing the potential threat of his foes rallying behind the legendary original Shield (recently unthawed from his stone like state), he framed the hero for a murder. Simultaneous, he recreated a fellow fiend named Eterno who quickly went from alleged automaton of the Emperor's to an even bigger threat. Realizing this, Brainy traveled to his old foe Comet's spaceship to enlist him in battle against their common foe. Little realizing another was following his dimension journey, who would soon unseat his plans.
Eventually, the combined forces of B.E. and the Crusaders defeat Eterno and his ersatz MLJer bodies. He continues to operate behind the scenes orchestrating a world wide network of crime under his influence, and yet eventually he is tracked down by his Mighty enemies including the Jaguar who captured the weapon used to frame the Shield. Left humiliated with tail between his legs, he assumed a more human disguise as the Dragon Head and employed a couple of agents,Eraser and Marital Law to attack the Fox while Emperor, under the guise of Brian Head, attempted to woo the heart of Fox's alias Paul Patton's girlfriend Delilah Monaco. And miserably failed both times. But the malcontent broke up the pair as revenge and further tormented them.
Brain Emperor then returned to his overt career as major league supervillian and enlisted the Riot Squad to deal with the Crusaders. Meanwhile, he banished the fair Darkling (the person who was tracking him above) to interdimensional limbo in a mindless state. He would one day return to wage war against the combined assemblage of virtually all his foes, using a legion of lethal larcenists. However, they ended up in an alternate reality that proved havoc to his alien physiology. So he just stole the body of local jock Moose Mason to lead his adversarial army to a futile final battle. His further activities are unknown. But the Jaguar is waiting.
The OTHER Brain is the one we were introduced to yesterday during E-Man's origin story. He is from Sirius One, sent as an advanced scout (or persona non-grata) of his race to unleash a bomb on Earth, preparing it for an impending invasion. Originally headed for Pluto, a certain sentient package of power from an ancient supernova impeded his journey, causing his craft to crash land on our fair planet, thus moving up his time table and his locale for heretofore mysterious further schemes vs E-Man.
The two future foes begin the first a long line of battles, the first proves E-Man the victor. Attempting to use a bomb to start a chain reaction that would make the Earth the perfect tool for the Brain's master plans, he failed to anticipate any obstacles. Despite superior mental acumen and technology. However, instead of caging the cagey cranium, the energized Alec Tronn leaves the ten-ton tumor in a field thinking he is permanently defeated. How wrong he was! The Brain realized he needed to enlist two youths that are the personification of Entropy.
While they wreak sizable destruction and nearly end the life of E-Man's main squeeze Nova, they are vanquished and then each Entropy twin is separated from the other by E-Man's positively charged blast which stymies the Siriusian.
And try again the Brain of Sirius would, several years late in 2006 which was 30 years from his initial debut. This time, he faced the combined might of two energy heroes, E-Man and the newly re-powered Nova Kane. He didn't fare much better, but until he is imprisoned once and for all he persists in his attempts to aggravate the life of the alien avenger.