A billionaire industrialist and a brilliant inventor in his own right, Tony Stark is the team leader. Following an incident that left him with a piece of shrapnel in his chest, Tony designed an electromagnetic harness to keep himself alive, and eventually, a suit of armor that could be powered by it.
Attack! Attack! Attack!: His plan to attack the HYDRA base? Hit everything hard. Later, he keeps pounding on the Super-Adaptoid while Cap's busy trying to tell him that it's not gonna work.
But He Sounds Handsome: Tony can't resist giving himself a pat on the back or two while disguised as the Grim Reaper and trying to infiltrate the Cabal. Comes back to bite him, though, as it makes MODOK and Skull suspicious.
Disability Superpower: It's hard to remember sometimes but that arc reactor in his chest is a glorified pacemaker. Remove the arc reactor and Tony's heart will very quickly give out. MODOK demonstrates this fact early on.
A sickly young man from Brooklyn turned into a perfect soldier by a government project during World War II. He ended up frozen in ice and awoken in the modern era during a battle with the Red Skull. His weapon of choice is a Vibranium shield that can absorb all energy directed at it, making it ideal as both protection and as a projectile (as it does not lose energy when it makes contact with another surface when thrown).
Berserk Button: Messing with his cape seems to be an open invitation for an ass-kicking.
The Big Guy: The tallest of the Avengers right after Hulk.
Boisterous Bruiser: Thor likes a good time; in the first episode, he's excitedly telling Hawkeye about an Asgardian festival he was missing out on. Then there's the time he and the Avengers went to a diner after defeating Hyperion.
Set Right What Once Went Wrong: Because he was in Asgard, and therefore in a different dimension, when Doom fired up his time machine in Planet Doom it is up to Thor to restore the timeline to it's original state.
He's big, green and likes to punch things. What more needs to be said? Formerly Bruce Banner, a leading scientist in the field of gamma radiation, he developed an alternate personality that manifested as the rampaging monster known as The Hulk after being mutated by said radiation.
Hidden Depths: His friendship with Glorium, being bowling buddies with The Thing, his glass figurine collection; these are not exactly things you would typically associate with a big gamma radiation-powered Blood Knight.
A brilliant, ruthless and power-hungry Nazi officer and prototype Super Soldier during World War II and the current head of the terrorist group HYDRA. The Red Skull is the Big Bad of the series, leading a Legion of Doom called the Cabal in addition to his own HYDRA forces.
The Bad Guy Wins: He pretty much gets what he wants in the first episode, "Blood Feud", and "Super-Adaptoid".
Badass: He's definitely a force to be reckoned with.
Cool Boat: His current base of operations is a rather impressive looking submarine.
Dangerously Genre Savvy: Realizes he needs help if he's gonna take down the Avengers. Plus, he tends to come out ahead in most of the episodes he's appeared in so far.
He also notices when Grim Reaper and Crossbones start acting more intelligently then they normally do, since they are really Cap and Stark in disguise.
Disability Superpower: The serum may have made him a Super Soldier but it also was slowly killing him. He tries to steal Cap's body to avoid death and when that doesn't work, he take's Tony's reactor and armor.
The Dreaded: Iron Man nearly has an Oh, Crap! moment in the first episode when Jarvis shows that Red Skull's still alive.
Enemy Mine: He and Dracula really don't like each other; HYDRA invaded Dracula's kingdom in World War II, but they have a common enemy in the Avengers.
Evil Versus Evil: He and Doom don't get along. Part of it has to do with the fact they're rivals for world domination. Another part might have to with the fact that Red Skull's an ex-Nazi officer and Doom's half-Gypsy.
Powered Armor: He steals Iron Man's armor at the end of the first episode.
Putting on the Reich: Most of the time we see him, he's wearing an SS uniform with the Hydra insignia replacing the eagle and swastika. Considering that he's actually an ex-SS officer, it's not too surprising.
Red Right Hand: Yeah, his disfigured face is a pretty good clue that he is not a good guy.
Weapons designer for HYDRA, their resident Mad Scientist and right hand man to the Skull.
Adaptational Badass: MODOK's shown to be more powerful than he usually is in other media. He took down Iron Man quickly, took out their jet and came within a hair's breadth of having them kill each other.
Evil Cripple: He is confined to what is pretty much a high-tech wheelchair.
Hate Plague: He uses nanobots to unleash one on the Avengers.
Insufferable Genius: MODOK has an ego even bigger than Tony Stark, no mean feat there, and little impulse control. He also bitter about the fact that the Skull is in charge and views MODOK as little more than a technical adviser and supply officer.
The King of the Vampire Nation, Dracula is centuries, if not thousands of years old. He fought with Capatain America against HYDRA and the Skull during World War II when they invaded his kingdom. But that alliance was only temporary, now he seeks Cap's blood as he believes it will make him immune to sunlight.
Don't You Dare Pity Me!: His reaction to Tony offering to assist in rebuilding the Destroyer destroyed area of Latveria, is to shock him, refuse, and tell the Avengers to get the hell off his land.
Day of the Jackboot: We get a good look at what New York City would be like if Doom had ruled it for a few years in Planet Doom.
Egopolis: He renames Earth Planet Von Doom after he conquers it and changes the Statue of Liberty to a statue of himself when he makes New York the capitol of his world empire.
Emperor Scientist: Granted, he plays around with magic just as much as he plays around with technology.
Evil Is Not a Toy: Invoked when he tries to summon and control the Midgard Serpent. Whether he would succeed in controlling him or not is left ambiguous at best. Later, it's shown that he did succeed in controlling the beast. However, he still went overboard with the Destroyer.
Rage Quit: So the people of his home planet rejected him and rose up against his totalitarian rule. His response? Blow up his homeworld and start looking for a new planet where he could establish himself as the top dog.
Son of the supervillian Molecule Man, Aaron Reece has recently acquired his father's invention: a device that allows the user to manipluate non-organic matter on the molecular level. He doesn't want to follow in his dad's footsteps and for most of his debut episode just wants to be left alone. Tony eventually convinces him to join SHIELD and talks Nick Fury into taking him in.
Cursed with Awesome: The wand only works for him and his dad, and while it's powerful, it's also wrecked his family, sent his dad to prison and put him in AIM, and by extension HYDRA and Red Skull's, crosshairs.
Cut Lex Luthor a Check: Reece Sr. invented what Tony Stark rightly called "the invention of the century" and the only thing he could think of using it for was to go on a crime spree.