At 4 years old, Lisa is the second-youngest child of the Loud family, and the second-youngest of Lincoln's five younger sisters. She is often seen working on complex experiments, equations, and formulas. Her most annoying habit is her weird studies on others, especially her siblings.
Before the beginning of the series, Lisa, despite her very young age, is a Nobel Prize recipient. She is still in kindergarten, but she is so much smarter than the other kids her age, that she can move up at least six grades.
Though the second-youngest of the Louds, Lisa is a child prodigy, so she's the smartest kid in the bunch. Because of this, she often tends to be egocentric and sarcastic with other people. She speaks with a lateral lisp, and usually has a stoic frown, though she does smile from time to time.
She is not as excited as her siblings, although sometimes she becomes in that state when something really good happens, like when she and the other sisters discover about Lincoln's romantic situation in "Heavy Meddle". She states this is because she does not partake in mundane human emotions as stated in "Heavy Meddle". She was also shown to have fallen for Hugh like all of her other sisters, and showed a more immature side to her, at one point having Hugh show off his six pack under the lie that she was teaching Lola human anatomy with him as an example. She also became somewhat irrational, claiming she would marry Hugh, and have children with him, despite the obvious age difference, and following him around like her other sisters. She also does not believe in superstition, fortune telling, blessings or curses as she is a realist and believes only in science as stated in "Raw Deal" and "Study Muffin". Nonetheless, she likes to be with her siblings and have fun with them, even if she doesn't express it.
In many episodes, she can be seen working on strange experiments. In "Left in the Dark", she creates an Oreo shaped cookie called "Gloweos" with the infused DNA of a jellyfish, and if someone ate it, all of his/her body will glow. One of her favorite hobbies is resolving very difficult math problems. She also likes to study the poop and bathroom habits of her siblings, something they all find gross and annoying.
As seen in "Overnight Success", Lisa not only likes to realize experiments with her siblings, but she also likes to realize them with people outside of her family.
Though easily the most mature of her siblings, Lisa is still a child not above childish plays like food fights. She has her own mischievous side. For the most part her pranks are hidden in unassuming objects; such as acne cream, sunscreen, mouthwash, etc, with results like turning skin blue, mild sunburns, and hot-pepper induced fever, respectively. Lisa is also completely willing to join in the madness caused by her siblings, whether it be disputes over money, the "sweet spot", or just playing along. This proves that while Lisa is a genius, she's still a kid and acts her age.
Contrasting most of her family, Lisa is very honest and blunt about what is on her mind. Lisa's brutal honesty isn't so much out of a sense of morality, but because she doesn't want wasted brain space by making up a lie. This also makes her incapable of keeping a secret. Like Lincoln she's very individual in contrast to rest of the sisters as seen in "It's a Loud, Loud, Loud, Loud, House".
Lisa, like her polar opposite Leni, is ironically very scatterbrained. She can frequently forget crucial ingredients to her various experiments, which can lead to devastating, though hilarious, outcomes. She also frequently forgets to look up while reading while walking, much to her embarrassment, and tends to misplace her vials, test tubes, and chemicals.
As seen in "Party Down" Lisa appears to have a fun side. As she gets a sugar high from having too much chocolate from a chocolate fountain. This proves that Lisa isn't a complete no nonsense person and can loosen up and act her age.
"Lisa Loud is smarter than the rest of her siblings combined. She spends most of her time in her lab setting off mysterious explosions, but she's always willing to help out with homework, or explain the structural flaws in someone's pillow fort. She loves her test subjects siblings!"
Lisa has short shaggy brown hair (which is a wig), and wears large glasses to help her see. She wears a green turtleneck sweater, pink pants, and brown shoes, all of which droop down to illustrate her stoic demeanor.
Her sleepwear includes a lime green nightshirt with matching pants. Her swimwear consists of blue goggles, and a green one-piece with a horizontal pink line in the middle.
"Potty Mouth" reveals that she is actually bald, toothless, and has an additional toe on her right foot. Because of this, she wears a wig and dentures.
Lisa's glasses are black-framed, with two circular lenses that are a translucent blue color, and only her pupils can be seen through them when she is wearing them. As of "Making the Grade", she has surgically corrected her eyes, and no longer needs them to see. However, she still wears them for unknown reasons.
In an alternate universe where the sisters are boys, Lisa is a boy named Levi. Like Lisa, Levi is a child prodigy, and speaks using really big words, as shown when stopping Leif and Lexx (Lana and Lola) from fighting, and telling them to be quiet, so Leon (Lily) can sleep.
Unlike Lisa however, Levi is shown to be harsh and uncaring towards Lincoln, as shown when the boys mock him after Luke (Luna) uses him to clean the bathroom, and teasing him over wanting a date with Ronnie Anne. He is also harsh towards Dad, as the boys plow him down whenever they see him. He also plays with the keep-away with the time watch his female counterpart made to send Lincoln home.
In another alternate universe where everyone is the opposite gender, and Lincoln is a girl, Levi is shown to be caring and honest with Lincoln, like Lisa.
Levi eventually ceases to exist, when it is revealed Lincoln's whole adventure was just a dream.
In the minicomic Deuces Wild!, Lisa interprets the superheroine called Card Counter, who is also one of Ace Savvy's sidekicks. She uses a special container to trap creatures like the gas monster. In the comic, she is summoned by Savvy (played by Lincoln) alongside the rest of Savvy's deck (played by the rest of her sisters) to help him and One-Eyed Jack (played by Clyde) to defeat the gas monster. After they defeat it, they prepared to fight against a garbage monster.
Lisa is the first of the younger Loud sisters to get an episode centered entirely around her.
Lori, Leni, Lucy, Lisa, and Lily are the only siblings not to have grayish-purple eyelids. In Lisa's case, when her eyes were shown, they were revealed to be black dots, due to her being nearsighted.
Lisa's sweater was originally blue before being changed to green in development.
For a long time it was thought that Lisa was the first Loud sibling to have their middle name revealed, in "April Fools Rules", where it's revealed that her middle name is Marie. However, it could be false. According to the "Ask Lincoln" segment in Instagram, their parents calls everyone Marie as a second name when they are in trouble.
This seems to allude Lisa Simpson from The Simpsons, as both Lisas have the same middle name and are the second youngest child in the family, as well as the most intelligent.
As shown in "Sleuth or Consequences", Lisa doesn't use the bathroom, because it steals time away from her experiments, and instead uses Lily's training potty.
She prefers to sit in the back of the car in case the engine blows up or a low flying plane comes in.
Lisa has already finished school up through getting a PhD (a type of doctorate degree), but in "Making the Grade," she's suddenly attending Lincoln's elementary school (albeit as a Kindergartener). Maybe she decided it would be best to go to school normally.
It is shown in "Driving Miss Hazy" that she doesn't mind doing Lori's homework, but doesn't appreciate how it's not "challenging" enough for her to do.
It is also revealed in "Study Muffin", that Lisa tutors all of their other sisters. She doesn't tutor Lincoln because she doesn't have the time to (although she apparently would tutor him if she was able to).
It is confirmed in "Picture Perfect" that she is almost blind and cannot see very well without her glasses (thus why she wears glasses).
She, along with Lincoln, Luan, Lynn, Lana, Lucy and Lily, are the only Loud siblings who don't normally have their ears pierced.
This changes in "Cover Girls", as the aforementioned characters, except Lisa and Lynn, have their ears pierced now, making Lisa and Lynn the only siblings to not have their ears pierced.
In the comic "Shocker", Lisa has the ability to understand Lily's babbling.
This means that Lisa, Lincoln, and Luan are the only ones who can understand Lily's babbling.
Lisa is the third most masculine and tomboyish Loud sister, after Lana and Lynn. She wears boyish clothes and doesn't care much about her appearance, but she is shown to have a slight feminine side, such as wearing pink, having an interest in going to the mall, being disgusted by Lincoln's grossness and enjoying operas.
Lisa is the second loudest of the Loud siblings, after Luna.
She is the only Loud sister who's eyelashes aren't visible, not counting Lucy who's eyes have never been shown in the series (as of yet).
She talks to the audience in "Undie Pressure", making her the first character besides Lincoln to do so.
In the episode "Snow Bored", it is shown that Lisa has connections with NASA.
She is the second-youngest, but the most intelligent of her siblings. Alternatively, Leni is the second-oldest but the least intelligent of her siblings.
According to "Party Down", she can get really hyped up on sugar.
It was revealed in "Spell It Out", that Lisa has spent 1,482 days in her lab, which equals to 4 years and 22 days. This means that she's been a science genius since her very birth.
Lisa, along with Luan, have broke the fourth wall the most, besides Lincoln, by doing it twice.
Lisa can sometimes be very immature and disgusting. As seen in "Study Muffin", she shows Lola Hugh's six pack and butt, and in "Lock 'N' Loud", she installed cameras in the bathroom, and she asked Lori and Leni for their stool samples.
In "Potty Mouth", it's revealed that due to a "nuclear experiment gone wrong", Lisa's bald (and has to wear a wig), and also has an extra toe on her right foot. Lisa's also revealed to wear false teeth, but it's never explained how (other than Lisa simply telling her siblings that it's a "long story").
However, the info about Lisa's hair and feet has been contradicted a few times in previous episodes, such as "Room With a Feud", when she is scratching her head, only to have her hair fall off, as if it was attached to her. Also, in episodes where she is shown barefooted, she doesn't appear to have an extra toe on either foot. It could also be that the accident happened recently, but didn't tell anybody about it. It's unknown if she'll make a medicine to cure herself.
Whether or not this is actually true or simply cartoon humor at it's most absurd is open to debate.
In "L is For Love", it is revealed that Lisa has a crush on a boy named David, who like Lisa herself, was born highly intelligent and enjoys studying science and other subjects. He also appears to be a male version of Lisa.
Lisa, Lola, and Lynn are the only sisters aside from Lily who have been topless on screen, in the episodes "Potty Mouth", "In Tents Debate", and "The Whole Picture" respectively.
According to Lori, Lisa took out Lincoln's appendix.
According to some information from The Loud House on Instagram:
She describes herself as "98% water".
She wants to travel to Alaska in the summertime for 24 hour daylight research.
No one has the clearance on her secrets.
Her biggest pet peeve is when a salad bar runs out of croutons.
↑Butterfly Effect/Script: LISA: "Flip's the only guy who will hire four-year-olds with no experience." REPORTER: "One has to wonder, where did it all go wrong for these girls? And why is a four-year-old working at a gas station?"