Note that this list also applies to the strip's spin-offs, Outland and Opus.
The star of Bloom County from its earliest days, Milo was the group's Only Sane Man, though he could be goofy as the rest of his friends when the situation called for it. Also a dedicated journalist with a habit for completly making up stories stretching the truth.
Almighty Janitor: Has a lot of pull at the Bloom Beacon despite being the intern and later copy boy.
It helps that the Chief Editor is a jittery wreck from trying and failing to hold off his yellow-journalism impulses, and thus spends much of his time hunched on his desk in agony as Milo temps him.
An anxious, loveable penguin and the main character for most of Bloom County and its two sequel strips (Outland and Opus). Opus is very nervous and often forced into situations he'd rather not be in; he is also a sucker for late night TV ads. He was also on a quest to find his mother for some of the duration of Bloom County and Outland, finding her at the end of the latter. But then he promptly fled in Opus when he found her insufferable.
Art Evolution: Opus' nose grows substantially larger as the series progresses.
Characterization Marches On: He also becomes less of a penguin and more like some kind of vaguely penguinish humanoid: his nose is much more like a mammalian/human nose (he even gets rhinoplasty at one point, and it's fleshy enough to liposuction) and he ends up with teeth and hands.
Butt Monkey: Forced into doing a lot of things and treated like crap by the world a good chunk of the time.
Breakout Character / Ensemble Dark Horse: Opus was originally only supposed to appear for a few weeks, but both the fans and Breathed himself loved the character so much, he quickly became a regular and eventually the main character. "Pear Pimples for Hairy Fishnuts" was arguably his breakout.
Interspecies Romance: Was engaged to human Lola Granola for at least a year and has gone on dates with other human women.
Laser-Guided Amnesia: After he and Cutter John disappeared over the ocean, he returned with regular amnesia. But after a while his normal personality and memories returned but what happened to Cutter John was unknown.
Luckily months later, a false claim that Eddie Murphy was marrying Opus' crush Diane Sawyer shocked him enough to jar his memory.
Missing Mom: A recurring theme in Bloom County and Outland.
Put on a Bus: Twice. The first time he and Cutter John were lost at sea and presumed dead and he came back with amnesia. Then later in the strip, after Bill became Fundamentally Oral Bill, he was kicked out of the Boarding House by the converted house members after Bill decried "Penguin Lust" on TV. He came back later after Bill stopped being popular and stayed ever since.
Wanting Is Better Than Having: Opus seached for all of Bloom County and Outland for his mother only to find out she was overly controlling and tried to force him to marry a large, replusive penguin named Eunice.
Bill The Cat
The second most famous character.
Anthropomorphic Shift: Became taller and longer-limbed over time. The trope was inverted as well, as Bill also gradually stopped speaking in anything but grunts and "ack"s and suffered a major drop in intelligence.
The Grotesque: Played with. Bill is an ugly, smelly, brain-dead cat, but he doesn't let this stop him from gaining fame, fortune, and women.
Hypocrite: During his stint as a self-righteous televangelist, Bill persecuted penguins for their "penguin lust." It's later revealed that he's had his own indiscretions with a "transexual slave trader". Bill showed no remorse, and his viewers quickly forgave him,
Why Do You Keep Changing Jobs?: He has been a lot of things, including a TV star, heavy metal frontman, dead, Medaow Party Presidential candidate, Rajneesh cultist, communist, televangelist and Donald Trump. (And that's just in the Bloom County canon...)
A wide eyed idealist and Milo and Opus' best friend. Lived with his dad Tom, who he clashed with with their polar opposite personalities. Also has a closet full of anxieties.
Progressively Prettier: In his very first appearance he was a gawky, chubby, bespectacled, bald kid. Though almost immediately he grew his iconic giant puff of hair.
Retcon: By the beginning of Opus, Binkley had apparently aged into a teenager, had a disastrous first kiss, and had become a Tibetan eunuch. He returned in a later comic, unaged, with zero explanation.
Earlier than that, near the beginning of Outland he'd apparently become Bart Simpson. This was likewise dropped.
A scumbag lawyer with murderers as his almost exclusive clientele, Dallas is a dedicated Republican, drinker, smoker, and womanizer. He's also Breathed longest lasting character, starting with his college strip The Academia Waltz. He is the only character to have the distinction of being a main character in every single one of Breathed's strips. note However it should be noted that Steve was not the only character to appear in all of Breathed's strips. Technically Cutter John was also in every strip, having co-starred Academia Waltz with Steve, and having appeared in various flashbacks and cameos in Outland and Opus. Later had his personality flip-flopped to become a sensitive guy, then flipped back.
Alcohol-Induced Idiocy: Steve's habit of having a few too many has led to incidents of this (most notably during his attempted "courtship" of Bobbi).
Suddenly Sexuality: In the very end of Outland Steve casually reveals he's gay, and runs away to get married to a Doonsebury character. Though with his return in Opus, he is back to his typical woman-hunting ways.
We Want Our Jerk Back: While he was flip-flopped for a long time, the cast were just as annoyed by his overly wimpy personality.
Hulk Out: When he drinks a mysterious substance and transforms into a towering hulk. The effects quickly wear off, however. Opus was pontificating with his back to Oliver the whole time and never noticed.
A 20-something feminist teacher who was a main character during Bloom County's early years. Eventually became less and less important until she up and disappeared from the strip. Dated Steve and then Cutter John.
Bus Crash: Was revealed to have later joined the staff on Donahue, and then cut her ear off when Geraldine Ferraro lost the vice presidency in 1984.
Moral Guardians: Threw a fit in Real Life over Lola's conversion to radical Islam. This led to that series of Opus comics being cut from several newspapers. The outcry eventually prompted Breathed to end the comic.