Varrick was always a brilliant and eccentric inventor and visionary, but he's also known for being a ruthless opportunist who will manipulate others and take any side to make it big. He played a big role in the Water Tribe civil war this way, and he spent time in Zaofu after that, learning the ways of metal and machines. In Season 4 of The Legend of Korra, Bolin and Varrick (with Zhu Li) signed up with Kuvira's Earth Empire to help restore the former Earth Kingdom, planting the seeds of personal change in all involved.
Creating The Opposition
Each season of The Legend of Korra has a unique villain who has unique abilities, minions and weapons to bear, and these villains threaten the entire world with these assets. In Season 1, Amon tried to rid the world of bending, and later, Unalaq tried to usher in a dark era while Zaheer sowed anarchy across the world. Kuvira, meanwhile, aimed to undo the chaos that Zaheer caused, uniting the Earth Kingdom's fragments into the newly-born Earth Empire. Advanced technology and humanitarian efforts alike drove this endeavor, and advanced tech means hiring experts like Varrick and Zhu Li.
This was Varrick's initial role in this season: to help Kuvira invent whatever she needs to solidify the Empire's control over the land. Already, Varrick had his own funds, as well as experience from Zaofu in the use of modern metal. Kuvira gave Varrick all he wanted: a state-of-the-art lab, Zhu Li's assistance, bottomless funding and a noble goal. It was a holiday for Varrick, and he eagerly helped Kuvira develop devices that could channel, store and control spirit power. But then Kuvira explained what all these gadgets were really for.
These were to become weapons, and Varrick decided that enough was enough. He'll get rich from war profiteering and create propaganda, yes, but giant cannons of destruction were a bit too much for him. Although Varrick and Zhu Li fled the Empire, Kuvira already had what she needed, thanks to Varrick. To become a true threat to the world and challenge Avatar Korra, Kuvira rolled out her giant spirit cannon, and attached it to a massive platinum colossus. With this weapon, she laid siege to Republic City, and none of it could have happened without Varrick's involvement in the Empire.
His Redemption
Varrick was unscrupulous from the start. He played both sides of the Water Tribe Civil War to get rich, and even escaped prison in Republic City and shamelessly fled to Zaofu to start over. He really just had an extended cameo in Season 3, but in Season 4, Varrick's "business without a conscience" method failed him. It should be noted that his loyal assistant and friend, Zhu Li, felt much the same way; she willingly took part in Varrick's war profiteering, but she balked at the idea of a giant spirit cannon. This put them at odds with Kuvira, but Varrick fled first, and Zhu Li stayed with Kuvira a bit longer to sabotage the spirit cannon project while feigning loyalty.
Varrick and Zhu Li reunited in Republic City. It had been a rough road for Varrick. He and Bolin had trekked across the hostile Earth Empire to find an escape route, and the journey had humbled them both and taught them what being a hero really means. Varrick gave up his mad ambitions and arrogance alike, learning that helping others is the most worthwhile project of all.
Once he came face to face with Zhu Li again, the latter stood up to him and declared that she would only work with him as an equal, and not a downtrodden sounding board. Varrick was taken aback, but soon saw things Zhu Li's way and reconciled with her. He had seen and done too much to bear the thought of exploiting or harming others. In fact, this newfound humility and humanity helped him realize his true feelings for Zhu Li, and out came the engagement ring. Zhu Li said yes, and their wedding was the capstone of Season 4's conclusion. In the end, joining Kuvira taught Varrick the consequences of ambition and showed him the virtues of humility and love.