Lori to her roommate Leni in terms of behavior. While they're quite similar in that both are beauty-obsessed teenage girl stereotypes who have blonde hair and care passionately about their popularity, their personalities are completely different from each other:
Lori has a strong sense of rationality and sophistication, which makes her overconfident about her ego and thus she can come off as quite rude, bossy, bratty, and demanding.
Leni is a complete scatterbrain who's incredibly naive and clumsy and has trouble thinking for herself even when in the face of the most rudimentary of tasks. Despite this, unlike Lori, Leni does not have a single mean bone in her body and is genuinely an All-Loving Heroine.
Leni and Luan have a lot in common in that both are perpetualoptimists and Cloudcuckoolanders who often get on others' nerves with their eccentricity but are Innocently Insensitive in their defense; however, Leni is a Valley Girl caught up in the recent trends, whereas Luan is a Kiddie Kid obsessed with comedy and little else.
Luna is this to both Luan and Lynn. While both of them are quite similar, they are also different in many ways.
Both Luna and Luan are brown-haired, showbiz-obsessed, kind-hearted, hammyGenki Girls who are Older Than They Look, wear skirts, and even have practically indifferent names sans the letters being rearranged; however, Luna is rather mellow, mature, and rational whenever not rocking out, while Luan is nearly always her cheerful, goofball self.
Both Luna and Lynn are brown-haired, rambunctious tomboys who can ham it up pretty good; however, Lynn is by default more jerkish, violent, and mean-spirited, while Luna is characteristically a Cool Big Sis with a mellow demeanor and good grip on reality.
Luan to Maggie from "Funny Business". Both girls are interested in pantomime and areas of performance arts. However, their personalities couldn't possibly contrast more strongly. Luan is cheerful, comedic, wears bright clothes and always enjoys spreading laughter and happiness everywhere. Maggie is a gloomy emo who wears dark clothes and her idea of a party is surrounding herself with like-minded people in what Luan describes as a 'mope-fest'. Maggie actively detests the idea of having fun, which is the very thing Luan lives for.
Lynn this to her sister, Lola—while they are pretty similar to each other in some ways (big egos, short tempers, displaying aggressive behaviors when they want something and/or are trying to get a message across) they're still pretty different from each other in other ways.
Lynn's easily the most masculine of Lincoln's sisters, deals mainly with sports/athletic activities and doesn't mind getting hands dirty (literally-speaking, that is)..
Lola's easily the most feminine of Lincoln's sisters, deals mainly with beauty pageants and prefers keeping her appearance clean.
Lana and Lola are Polar Opposite Twins in terms of femininity and kindness- Lana's a tomboy, Lola is a pageant-obsessed princess. However, while Lana is easily one of the kindest siblings, Lola has a Hair-Trigger Temper and is basically a brat.
The Loud brothers from "One of the Boys", especially the older ones, to the Loud sisters. They do share some traits in common, but the brothers are overall far more rude and aggressive towards Lincoln (barring Lynn, who is basically the same across genders) while the sisters are overall supportive of him.
Loki to Lori — He is the oldest sibling, but not responsible like Lori and is more willing to join in the antics of his younger siblings. Loki also enjoys teasing Lincoln's relationship with Ronnie Anne rather than be a Shipper on Deck.
Loni to Leni — He is a Dumb Blonde like Leni, but unlike Leni, he is not sensitive and does not care when his brothers get hurt and laughs at Lincoln's relationship with Ronnie Anne and when he suffers physical pain.
Luke to Luna — Unlike Luna, who is a Cool Big Sis, Luke is a Big Brother Bully who gives Lincoln a Swirlie and is the one who flushed Bun-Bun down the toilet.
Lane to Luan — He likes to do jokes and pranks, but his tend to be generally more humiliating and mean-spirited than Luan's. He gives Lincoln a wedgie for no good reason while Luan, outside of April Fools, rarely uses violent humour and prefers to make puns and use her water flower.
Though he and Lincoln do have a lot in common, Clyde's an only child in contrast to Lincoln, who comes from a family of eleven children. In addition to that, Lincoln lives in a home with mostly females—not counting any of the Loud family pets, Lincoln's dad is the only other guy in the family. Also, Lincoln and his sisters and both of their parents (who're husband-and-wife) are all white. Clyde, on the other hand, is black, is the son of an interracial same-sex male couple (Howard is white, Harold is black), and the only known females in their household are their pet cats, Cleopawtra and Nepurrtiti.
Howard and Harold are also foils to Rita and Lynn Sr., even sharing similar clothing styles. While the Loud parents are used to raising 11 kids, their time is often stretched thin among all the kids to the point of said kids wishing for undivided attention. Meanwhile, the McBride dads have only one kid (Clyde) and thus dote on him more than Rita and Lynn Sr. do to their kids to the point of overprotectiveness, while also getting exhausted from dealing with the Loud kids in a single afternoon.
Rusty Spokes to his brother, Rocky Spokes. Despite all his efforts to be considered a cool, popular ladies man, Rusty is ultimately part of Lincoln's gang of geeky friends. Meanwhile, his brother Rocky is a (mostly) normal, cool and laid back kid who has typical interests for a boy his age like playing sports. In his earlier appearances, Rusty was shown to be hanging out with other kids, but he ended up having only Lincoln, Clyde, Zach and Liam (and later Stella) as his only friends. Additionally, Rusty is a bit of an excitable wuss and is Afraid of Blood, while Rocky shares Lucy's interest in blood, horror, and the macabre.
Darcy Homendollar is a foil to Lisa Loud in everything - while Lisa is a Child Prodigy, Darcy is a typical preschooler who doesn't even know how to read yet, and while Lisa is a stoic introvert who isn't very good at socializing, Darcy is a cheerful extrovert who loves making new friends.
The Mccauley Kids to the Fox kids, obviously. The Fox Quintuplets are little hellraisers who love to roughhouse and be very naughty and mischevious. The Mccauley's are far more behaved and good-natured, and don't like Lynn's rough ways of babysitting, unlike the Fox's who love it.
While she and Leni are friends, Fiona is more confrontational and easily angered while Leni is a kind-hearted All-Loving Hero.
Miguel to Bobby Santiago. Both of them are light-hearted Hispanic teens who enjoy feminine hobbies and have close relationships with the eldest Loud girls. The difference between the two is that Miguel is shown to be more critical and direct when interacting with other people while Bobby is more soft-hearted and bends over backwards for others, especially his friends and loved ones.
Lacey St. Clair to Lola Loud. Both are girly girls compared to Lana, and both have competitive natures relating to their fields of interest (pageants for Lola, fancy dog competitions for Lacey). But Lola genuinely loves her sister and has a heart of gold behind all her negative behavioural traits. Lacey is malicious and doesn't really come across as so redeemable. Lola accepts Lana the way she is and even willingly hangs out with her (for the most part, as sometimes their clashing personalities can lead to conflict), while Lacey looks down on Lana and antagonises for the fun of it.
Ronnie Anne to Bobby. They're siblings and love interests to the Loud siblings, but they differ in how they treat their lovers- she treats Lincoln roughly, while Bobby treats Lori with the utmost respect. Also, Bobby is a Kindhearted Simpleton, while Ronnie Anne is actually pretty smart.
Carlota Casagrande to Lori Loud. Both are the eldest child and both are left in charge of their siblings when their parents are out. However unlike Lori who manages to control her siblings, Carlota has no control over her younger brothers and spends her entire time babysitting them being pranked.
Hector Casagrande to Flip. While he is motivated by profit in the same way Flip is, at least Hector runs his Bodega honestly and builds rapport with his customers.
