Justice League United (Season 1):

We could say that T.O.Morrow/Thomas Oscar Morrow was the main antagonist & made some improvements on Tornado (made his appearance a bit more heroic), made modifications to his body so he can now assume human appearance with a conscious act of will to infiltrate the D.E.O. 
Red Tornado
while Red Tornado tries to play hero & turn public opinon against Supergirl & created Reds Torpedo & Inferno. A few episodes later, Supergirl & her team figures out that John Smith & R.T. are 1 & the same. Morrow then lured Supergirl into a trap & just completed his new creation "Red Volcano". When Morrow ordered R.T. to kill Supergirl, she reminded him of all the good he's done. Taking these words into mind, R.T. turned on Morrow. After the battle, Supergirl brought a heavily damaged R.T. back to the D.E.O. for repairs. Kara & her team questions Tornado's previous intention, R.T. said: “Morrow created me to be a hero, and he thought that meant less human. To be cold and unfeeling. I wish to be a hero, but I believe that means being more human". From then on, R.T. has served Supergirl & the J.L.U. as a team member. During the season, Superman/Kal-El/Clark Kent, Steel/John Henry Irons & Red Tornado/John Smith will join the League.

Justice League United (Season 2):

We could say that this season's main antagonist is Mongul. According to the comics, Mongul is the tyrannical ruler of Warworld, an artificial satellite that traveled the galaxy picking up slaves for gladiatorial combat. We could've said that Mongul had Superman & J'onn kidnapped to have them forced into his gladiatorial combat. Guardian/Jim Harper, Miss Martian/M'gann M'orzz/Megan Morse & Brainiac 8/Querl Dox would've joined the League.

Justice League United (Season 3):

We could say that this season's main antagonist is Despero. According to the comics, Despero is an intergalactic conqueror & the ruthless tyrant ruler of planet Kalanor, he possesses incredible mental powers in addition to his great strength and intellect. Agent Liberty/Ben Lockwood, Insight/Mastrocola & Dreamer/Nia Nial would've joined the League.

Justice League United (Season 4):

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