His physical acting is just fine though - in "Brain's Song", he bounces a lemon-football around like a basketball during a live broadcast, and in "Cinebrainia" he essentially becomes a silent-film comedy actor this way.
Berserk Button: Downplayed in the sense that he doesn't go blind with rage, but there are two things that he will give top priority to - saving Pinky, and stopping Snowball.
The Determinator: Tonight's plan failed? Start planning for tomorrow night. Repeat as needed.
Ditzy Genius: His plans are quite brilliant, but lack common sense.
Evil Genius: Averted, in a couple of ways - he is a genius, but Brain works for himself and was never particularly evil (he'll take direct action against actual evil) - but we don't have "Anti-hero Genius" as a trope yet.
Even Evil Has Standards: His refusal to sell his soul for world domination in the Halloween Special. Usually an aversion, since he was never particularly evil to being with, and has enough scruples that he doesn't need Pinky to be a Morality Pet... most of the time.
One Very Special Episode against smoking has Brain playing Brainy The Elf, a spokesman for the tobacco industry, who use him as a mascot to get children to smoke. While he initially planned to put an additive in the tobacco as part of a plan to take over the world, he has a My God, What Have I Done? when he sees children about to start smoking, and turns against the tobacco industry.
Fatal Flaw: His lack of common sense. He has a few other glaring flaws, but this is what kills the nightly plan the most.
Freudian Excuse: Being taken from his home and family when he was younger. Where were they living? A can with a globe (ie, the world) on it.
As a lab mouse, he's subject to testing; in "Where No Mouse Has Gone Before" at least, he holds a mild grudge over this.
Friendly Enemy: At least tries to be this to Snowball. It never works though.
Gadgeteer Genius: Brain's strongest suit. Quite often, the gadget or means to world domination is assumed to be an automatic success, and their 'twilight campaign' is typically the duo filling in a minor cog in the plan.
It also only applies to gadgets - take away his lab access, and he's completely lost.
Glad I Thought of It: Averted...? Though he will criticize the original comment as being unhelpful or irrelevant, quite a few plans are Brain's extrapolation(s) of something Pinky said, and he's more than happy to dole out credit for it.
Guile Hero: ...Hero might be pushing it a little, but he shows a clear preference for plans involving as little physical violence as possible, with (temporary) Mind Control and other forms of influence being favorites.
Hoist by His Own Petard: The nightly plan usually fails because something he did boomerangs on him, such as trusting Pinky with a crucial part of the plan (more common early on), or some minor detail turns out to be a lynchpin.
Indy Ploy: Averted. Brain's not all that good at dealing with a Spanner in the Works he didn't account for (separate from his lack of common sense), though this is due to an element he couldn't have predicted as often as it is just him dismissing something as irrelevant. In some cases, the plan gets as far as it does on sheer dumb luck.
Insufferable Genius: Usually downplayed, at least on the 'insufferable' side. He's certainly a genius, but not a braggart, and he understands people well enough to give a good impression.
Jerk with a Heart of Gold: It's implied that he does have a small soft spot for Pinky deep down. And by 'implied', we mean 'all but publicly stated with a megaphone from the rooftops'. (It's most obvious when he thinks he might have accidentally hurt or killed Pinky through a device.)
Oh, he definitely cares. Multiple times he has stated his conquests as "our", and though he repeatedly snarks at Pinky he would go out of his way to help him, even if it means aborting every single plan.
Kavorka Mouse: Has no trouble attracting women when playing a human.
Sarcastic Confession: Quite often, Brain will be forthright with people that he and Pinky are lab mice scheming to take over the world. Whoever he says this to thinks he's just joking around (and that's probably what he intends).
Sesquipedalian Loquaciousness: He can speak in layman's terms when he wants to (he switches over when dealing with people as part of the nightly plan), and Pinky understands him just fine most of the time. It's only gotten in the way of a plan when he tried to be a stand-up comedian - even then, he became popular as a highbrow insult comic.
This was a bit of Character Development on Brain's part; the notion of making the world better was a result of events in the episode "Megalomaniacs Anonymous"...and even then, he's still doing it just a little for his own ego.
Even before that, there's every indication that he plans to rule the world far more benevolently than Snowball.
There are even quite a few implications that if he succeeded the world actually would be better off — in the Freakazoid! episode "Freakazoid is History," Freakazoid inadvertedly changes history and finds himself in a world where the Brain is president of the United States and the world is better off for it — among other things, cold fusion works and Sharon Stone is a good actress.
Break the Cutie: The Christmas episode, The Halloween Episode, That Smarts and The Pink Candidate to name a few episodes. He can easily go from Plucky Comic Relief to Woobie under the right conditions.
Bumbling Sidekick: Though Brain clearly trusts him as an assistant, and they both think of it as a partnership/friendship anyway.
Bunny-Ears Lawyer: There are hints that Pinky is actually very intelligent, the genius in the opening theme song, but his poor connection with reality and the fact that he's quite content with his lot in life hides it too well.
Genius Ditz: The theme song doesn't specify which one of them is the genius or insane... But in a more literal sense, Pinky's wide-ranging knowledge of pop culture (current and past) comes in handy from time to time - and he supplies them with no hesitation whatsoever.
Hidden Depths: Most of the time he's goofy and hyperactive, but has a strong moral compass that shows on occasion. Particularly when he became president of the U.S for one episode and turned Brain down when he tried to get him to abuse some of his new powers because "it would go against everything I've come to stand for."
Besides that, while he doesn't have Brain's sheer intelligence, he is still keenly observant and humanistic, and by far the more grounded of the two.
You wouldn't think being easygoing and adaptable would ever be useful, but it saves their lives in "Welcome to the Jungle" - Pinky manages to come up with stone age-level solutions to being trapped outside, while Brain is reduced to complete reliance.
Indy Ploy: Pinky's fast talking has been the only thing keeping the plan going on many, many occasions.
The Load/The Millstone: On occasion, though Brain's plans tend to fall apart without outside help. Early on in the Animaniacs segments, he tended to be the latter far more often; by the time of the series, he's rarely even The Load.
Brain at one point makes a calculation that it's always Pinky's fault that his schemes fail, so he uses an intelligence-enhancing device to make Pinky smarter. Pinky uses this newfound intellect to point out that it's Brain's nonsensical plans that are doomed to fail in the first place.
A hamster who was put through the same gene splicer as the mice and also schemes to Take Over the World, but does not intend on making it a better place like Brain would. As such, he is The Rival to the Brain. Was eventually changed back to an ordinary hamster in his last appearance. Voiced by Roddy Mac Dowall.Tropes associated with Snowball:
Love Dodecahedron: Between her, Brain, Pinky, Pharfignewton and Snowball. Specifically, both Brain and Snowball love her, but she has a crush on Pinky, who in turn is in love with Pharfignewton.
A grey and white race mare (female horse) whom Pinky fell in love with in the segment "The Kentucky Derby". She reciprocates his feelings, but sadly, they come from different worlds... not to mention different biological orders. The relationship between them obviously Squicks Brain.Tropes associated with Pharfignewton:
A dopey chubby mouse with frizzy hair that sticks out in the sides like Larry Fine of the Three Stooges. He accompanied the duo during one of their schemes, doing little except constantly introducing himself and providing useless observations. Eventually, Brain tires of him and tells him to leave. He later joins up with Paul Simon. He was voiced by Billy West.Tropes associated with Larry:
Here We Go Again: Finally, Brain managed to get rid of "Larry", and be just a duo with Pinky again. Now it's time to take over the world, without Larry... and the scientist opens the cage and introduces a new mouse, Zeppo. And Here We Go Again
Dr. Mordough's cat who was the Evil Mastermind of the scheme the mice had to foil in "Brainwashed". Like the mice, she too gained brainpower from the gene splicer, but lost her feline beauty and swore vengeance on humanity as a result. Voiced by Nora Dunn.Tropes associated with Precious:
A humanoid alien who scouts planets to eat the being with the highest intellect's brain, he attempts to steal Brain's after sending him an email promising to help him take over the world, after his defeat he decides to settle for Dick Cavett's. Voiced by Jeff Bennett.Tropes associated with Zalgar:
A fat slob who replaced Montana Max as Elmyra's love interest in Pinky, Elmyra and the Brain. Has the hots for Brain's occasional alter-ego, Patty Ann. Voiced by Nancy Cartwright.Tropes associated with Rudy:
Ambiguously Gay: He must be this if he's willing to fall for a mouse-haired girl who speaks with a clearly male voice.