He dislikes whiny students and Friday nights. Though he is incredibly unsatisfied with his job and is often trying to seek a new one, he wants to have a life-changing affect on his students. He is shown to teach multiple subjects, as shown in different episodes. As frustrated as he is with his students from time to time, he still cares about them, and sometimes takes parental approaches to disciplining them, such as calling students by their full names when they are misbehaving ("Happy Birthfish, Jocktopus," "Brothers' Day," and "Milo and Oscar Move In"). He sometimes becomes empathetic with them, like when he associates Milo's declaration of Brothers' Day with his own desire to have one brother, rather than eight older sisters, in his youth. Even though he never knew he could get pregnant, he seems to have accepted it quite well, especially after revealing it to the class, and is even open about it from time to time.
According to Milo, his nose looks like a giant sea cucumber.
He was first seen when he spotted Bea sneaking into Clamantha's locker so she could get her yearbook picture. When the "Grow-Dino" grew and exploded inside the locker, he simply said, "Works for me" ("Bea Stays in the Picture"). It is shown that Mr. Baldwin sleeps during his job. Mr. Baldwin does roll call every morning, and in the episode "Fish Out of Water", he can't stand Milo's entrance with the eel. Mr. Baldwin says it can't be about him all the time. He then reveals that he's pregnant and never even knew male seahorses could get pregnant ("Fish Out of Water"). He caught Jocktopus beating up Albert and called him by his full name. He then made Albert go to detention for standing in the hallway too long ("Happy Birthfish, Jocktopus").
When Mr. Baldwin did a lecture about Shellington, Milo was causing several distractions. When Baldwin could not take it anymore, he told Milo to see him after class. He put him in a bag full of water and told him he was failing class. Milo assumes he is joking, and heads off, leaving Baldwin annoyed and dumbfounded. When it was time to take the test, Mr. Baldwin noticed Milo wasn't in his desk. When Milo barely passed the test, Mr. Baldwin was as overjoyed as Milo, as he wouldn't have had to hold him back another year and "lose any more hair" ("Fail Fish").
Mr. Baldwin once took Oscar's backpack, and when he picked up his phone a moment later, he told his students he had "car trouble". Milo thought Baldwin was secretly Captain Aquarium, so he, Oscar, and Bea (who needed a story for journalism) spied on him to see if he was a superhero. At first, the book said Captain Aquarium saved the city from giant monsters, and Mr. Baldwin appeared to be doing it, until it revealed he was just getting new glasses. Afterwards, the book said Captain Aquarium fought criminals, but Baldwin ran away when he saw criminals at the bank. Lastly, it said Captain Aquarium changed his outfit in a phone booth, but Baldwin just napped there. Milo still didn't want to give up, so he came up with a plan. Bea led Mr. Baldwin to Oscar, who was dressed as a villain, and Milo trapped in a glass cup. Baldwin didn't fall for it and was about to take Milo and Oscar home, but when Bea flashed her camera, Mr. Baldwin accidentally let go of the cup. The gang escaped the sharks, as they were vegetarians, and Mr. Baldwin helped Bea pass journalism ("Baldwin the Super Fish").
Mr. Baldwin once did a drawing of a robot fighting a dinosaur on the board, but was interrupted by Albert's coughing. He then noticed Albert had the dropsy and hit a button, making Principal Stickler and Nurse Fishington come and put Albert in a bag ("Dropsy!"). When the Swirlies were about to come on, Mr. Baldwin wanted tickets and asked his students if they had any, even offering them an A for the semester. Albert stood up, but it just turned out to be his birthday and he was gonna hand out cupcakes ("Fishing For Compliments: The Albert Glass Story").
When Milo turned giant and smashed Mr. Baldwin's classroom's wall, Mr. Baldwin said "I'm not even gonna turn around," and continued drinking his coffee. When Milo turned extremely giant and tried to climb the tanks to Freshwater High, he passed Mr. Baldwin's apartment room. The moment Mr. Baldwin saw Milo, he froze in shock and his hair fell off ("Big Fish"). When Milo went out peopleing, Bea suggested Milo find some other cool role model. She suggested a fish teacher, and they saw Mr. Baldwin sucking algae off gravel ("Peopleing"). When Milo went crazy for recycling, he took off Mr. Baldwin's glasses while he was driving, causing his car to crash ("Legend of the Earth Troll").
When Milo asked Mr. Baldwin why male seahorses could get pregnant, Mr. Baldwin didn't respond ("Good Morning, Freshwater"). When Bud was cleaning the tank that included Freshwater High, Mr. Baldwin told everyone to escape, and threw Clamantha in the classroom afterwards to see if it was clear ("Diary of a Lost Fish"). When the class went on a field trip to Pupu Goodtimes Amusement Park, Mr. Baldwin was shown to hate chaperoning field trips, and told the students to bother him only if it was an emergency. When Mr. Baldwin was about to go on the Pu-Panator, the ride attendant told him that pregnant seahorses weren't allowed to ride ("Good Times at Pupu Goodtimes").
When Mr. Baldwin was taking attendance one, he came across a couple of student's name that started with an "O", and Oscar came to school just in time for his perfect attendance award ("Run, Oscar, Run"). Mr. Baldwin was shown to be really bored, and even slept at some points in the "Potatoes for Winter" play. When the play was over, Mr. Baldwin was still sleeping, eventually waking up, glad that the play was over ("Fish School Musical"). Mr. Baldwin gave Milo and Oscar all of his money for a hat so Milo and Oscar could stop begging him to do so. He then ran away from the store, screaming. At the dance, Baldwin gave Ms. Lips the hat that he bought, and said, "What a doll" when he was watching her skip into the school, showing his first hint at a crush on her ("Employee Discount").
When Mr. Baldwin got Steve Jackson's hair gel all over his hair, he showed it to Milo, Oscar, Albert, Jumbo, and Steve Jackson, but they seemed really disgusted by his hairstyle ("Hairanoid"). Bea and Milo delivered Mr. Baldwin's present to his house on Christmas, causing Baldwin to wake up the second they left thinking they were intruders ("Merry Fishmas, Milo").
In "Fish Lips Sink Ships", he reveals his crush on Ms. Lips to Milo, after he is caught rehearsing asking her out. His primary reason for never doing it before was out of paranoia that he would be rejected. While trying to stop Bea from revealing his crush to Ms. Lips, he finally works up the nerve to ask her out.
Mr. Baldwin and Ms. Lips are shown to be in a relationship after the episode "Fish Lips Sink Ships". In "Labor of Love", Mr. Baldwin follows Ms. Lips to Flip Flop Island, trying to work up the courage to ask her to marry him. With the help of Milo and Bea telling him what to say through a headset, he tries to ask Ms. Lips to marry him, failing when he accidentally calls her cat stupid because of Bea. Ms. Lips gets angry, and Mr. Baldwin tries to apologize, only burping because of his nausea. Ms. Lips smacks him, sending the engagement ring deep into the temple they were standing on. Later, Mr. Baldwin is shown depressed, eating lots of tacos. Milo and Bea get everyone together to try to set Ms. Lips and Mr. Baldwin up without Ms. Lips knowing until she sees him. They send Oscar, Jumbo Shrimp, and Albert after the ring. After Ms. Lips arrives at the restaurant Mr. Baldwin is at, she is at first disappointed he is there, but starts to become a little fonder of him after Milo and Bea put on a play about how Mr. Baldwin likes cats. After Bea accidentally calls Ms. Lips cat a "dumb furball", Ms. Lips starts walking away again. Mr. Baldwin stops Milo and Bea from creating a new plan, and decides to take matters into his own hands. He starts singing "Please Say You'll Stay", while Oscar, Jumbo, and Albert finally get the ring. The cat that was keeping the ring chases them, and they burst into the restaurant in the middle of the song, but Mr. Baldwin continues singing. He finally proposes to Ms. Lips when he catches the ring from Milo, and Ms. Lips is about to answer when Mr. Baldwin suddenly goes into labor. The cat that was chasing them calms down and tells Mr. Baldwin how to deliver his babies. After Mr. Baldwin tries and tries, he finally gives birth to about 100 to 200 offspring. Ms. Lips reveals afterwards that she will marry him, and they go home.
Mr. Baldwin shows dislike towards Milo and doesn't generally appreciate him. He thinks of Milo as a "selfish fish" and also thinks it's annoying how Milo always interrupts the class when it's "learning time" ("Fish Out of Water", "Fail Fish"). As much as he is irritated with Milo, he cares about his welfare, and shows great concern for him (and himself) when he is failing the class, as well as making an effort to rescue him while falling from the edge of a shelf ("Baldwin the Super Fish"). He loses hair because of his stress teaching Milo ("Fail Fish"). He seems to trust Milo very much when he tells him about his crush on Ms. Lips ("Fish Lips Sink Ships").
Milo always cares about Mr. Baldwin, and helps him when he needs a break or a hug. He is like Milo's role model. ("Fish Lips Sink Ships"); ("Labor of Love"); ("Hats Amore!"); ("Don't Let the Fish Drive the Party Bus").
Mr. Baldwin does tend to show a bit of care towards Milo in episodes like "The Big Woo", when he joyfully pranks Milo back, and seems proud of him graduating.
- He is voiced by Dana Snyder, who also voices Bud and Clamantha`s Mom.
- He often sleeps on the job.
- Mr. Baldwin's first name was never revealed at any point in the series.
- He gave birth to one hundred to two hundred children in "Labor of Love", instead of the quadruplets mentioned in "Spoiler Alert".
- He never knew that male seahorses could get pregnant until it happened to him ("Fish Out of Water"). In spite of this, he at times seems eager to become a father.
- He has a "Baby aboard" sign on his car.
- His car license plate is "PREGDAD1" ("Legend of the Earth Troll").
- He has a best friend named Mark, who has never appeared in any episode of the series ("Dropsy!").
- Milo said during his class that his nose looks like a giant sea cucumber ("Fish Sleepover Party").
- He has a "Panic Button."
- Mr. Baldwin hasn't been out of his house on a Friday night since 1986 ("Baldwin the Super Fish").
- He has a very strict "no sleeping in class if he doesn't get to" policy ("Milo and Oscar Move In").
- He has eight older sisters, who have never appeared at any point in the series ("Brothers' Day").
- He goes to the restroom frequently when he's pregnant ("Labor of Love").