Unalaq

Unalaq is a major antagonist in The Legend of Korra, appearing as the secondary antagonist of Book 2: Spirits (alongside with the main villain being Vaatu) and a posthumous antagonist in Book 3: Change.

Unalaq is Korra's uncle and second archenemy, the younger brother of Tonraq (Korra's father), and the father of the twins Desna and Eska. Though he initially seemed to be a well-meaning individual who offers to train Avatar Korra in the ways of the Spirits, his villainous side was revealed when it is learned that he was the one responsible for the banishment of Tonraq from the Northern Water Tribe.

Bad Qualities

 * 1) Compared to all of the other antagonists throughout Legend of Korra who were at least entertaining and compelling, he seems like a bland, cartoonishly evil antagonist.
 * 2) *Amon from Book 1 was unique for being one of the few people who could take away someone's bending, and he was fighting for equality thinking benders were a dangerous threat and making some rather valid points.
 * 3) *Zaheer from Book 3 featured a complex philosophy about a world free of tyrants, exploitation, and oppressive laws. On top of that, he delved into the spirit world a number of times and he was an acrobat air bender, who fought with a Lavabender, another Combustionbender, and a terrifying Waterbender. Plus, he's crazy unpredictable. How awesome is that?
 * 4) *Kuvira from Book 4 was a master strategist and an elite Metalbender with an incredible large-scale army and some cutting-edge tech. She also really wanted to reclaim the United Republic to make the Earth Kingdom into a coherent whole and restore order and prosperity.
 * 5) *Vaatu was a good villain on his own, being a powerful spirit, but Unalaq stands alone as being a generic rotten-to-the-core baddie with zero sympathetic qualities or coolness factors, which makes him feel unoriginal and inferior in every way rehash of Ozai from Avatar: The Last Airbender.
 * 6) Squeaking of rehashing. He also acts like a mostly weak foil to Gyatso from Avatar: The Last Airbender. Due of these two are having connections (artificial and/or biological) with the current Avatars (Gyatso being a Airbender Monk/Member of the Council of Elders and father-figure to Aang and Unalaq being a Waterbender Monarch and uncle to Korra). And have different beliefs of how to connections with spirits (Gyatso is non-radical and peaceful, while Unalaq is the opposite).
 * 7) He has a generic look that looks too much like Tarrlok, making them very difficult to tell apart.
 * 8) His motivations regarding uniting the material and spirit worlds aren't fleshed out, and his development feels rushed in no small part to his characterization mentioned above.
 * 9) Also his plan makes no sense. He wanted to release Vaatu (the spirit of darkness), merge with him, and bring about 10,000 years of darkness. Why he thinks this is a good idea, and how ending humanity is supposed to help his earlier goal of restoring his nation's neglected spiritual beliefs, is not really addressed. But hey... evil!
 * 10) His constant flip-flopping over whether he needs Korra to open the northern spirit portal is pretty awkward.
 * 11) His manipulation of Korra would've been more effective and lasting, if he hadn't issued a civil war on the North and South Water tribes and tried to do away with his brother, Korra's father, just out of petty spite.

Good Qualities

 * 1) His voice actor, Adrian Latourelle, did a good job voicing the role.
 * 2) Though poorly executed, the idea of having a Dark Avatar as opposed to its Light counterpart is a pretty cool concept.
 * 3) Unalaq started off interesting as a well-intentioned villain, though he rather turned into a generic Doomsday villain quickly.
 * 4) He is capable of spirit bending which can be pretty cool.
 * 5) His death in the finale was satisfying to watch.