According to an Instagram post from the production crew, Clyde is Howard and Harold's adoptive child.
Clyde is awkward, caring, and nerdy, but sweet and supportive at the same time. Lincoln can always count on Clyde to help him with a plan. Like Lincoln, Clyde has a passion for comic books, video games, and science fiction movies. He is also a cat lover and owns two cats that he loves very much, despite how his oldest cat Cleopawtra always misbehaves.
Clyde is quite intelligent beyond his years. He knows how to do taxes and about table manners as seen in "A Tale of Two Tables" and knows how to take care of babies. In addition, he seems mildly rich through his parents, given his nice house and lifestyle as shown in "Attention Deficit" and "Roughin' It". He is socially inept, but ultimately a very friendly and kind person. Clyde is patient and enjoys the company of all of Lincoln's sisters, since he gets lonely with being an only child.
Clyde is very neurotic, cowardly, cautious, and worrisome. He has asthma, which requires him and his dads to carry an inhaler everywhere. According to his parents, he cannot consume meals with too much sugar or orange juice with pulp. Clyde is allergic to all kinds of nuts, especially peanuts, as well as gluten (possibly non-celiac gluten sensitivity). "Dance, Dance Resolution" shows that he suffers from acrophobia, a fear of heights, and can show severe signs of altitude sickness because of it. Clyde often panics and overreacts; when he does, he hyperventilates into a paper bag that he carries with him at all time.
Clyde is a huge fan of Blarney, the show's version of Barney the Dinosaur as seen in "The Price of Admission". He knows all of Blarney's songs by heart. He also owns many stuffed animals that keep him company. Clyde has his own laptop and a white noise machine in his room. "April Fools Rules" shows that he has a baseball trophy, possibly passed down from Harold (who used to be a baseball player).
A running gag on the show is Clyde's unhealthy and obsessive crush on Lori. Seeing, hearing, or even thinking about her causes him to freak out, bleed profusely through his nose, and act like a malfunctioning robot in some cases. He occasionally faints as well. In his closet, Clyde has a shrine filled with candid pictures of Lori and treasures things she gives him or has touched or he has stolen from her. Clyde also sends Lori gifts, as seen in "Change of Heart", but Lori stores Clyde's gifts in a box. Sometimes, Clyde tries to sabotage Lori and Bobby's dates, but always fails and ends up making their love stronger.
"Clyde is Lincoln's best friend and wingman. As an only child, Clyde is jealous of Lincoln's full house and is always willing to play his part in a crazy scheme. He has a big crush on Lori and can't act normal when she's around. Clyde spends so much time at the Loud House; he's almost an honorary sibling!"
Clyde wears circular glasses with thick black rims and blue-tinted lenses. He has freckles and black hair styled into an Afro. Just like Lucy, his hair always has a bright outline (although his is white and Lucy's is blue). He is normally seen wearing black pants, and a yellow-and-blue striped shirt with a white collar. He wears black tennis shoes with three white stripes, and white laces on each.
Clyde's sleepwear consists of a blue nightshirt with a white collar, and a pair of matching pants. Howard has mentioned that Clyde also owns footsie pajamas.
Clyde wears glasses. They look nearly identical to Lisa's in design and color scheme. However, Clyde's glasses are somewhat more oval-shaped and less thick. The color of the lenses is a lighter shade of blue than Lisa's.
At the end of "One of the Boys", Lincoln is sent to a second alternate universe in which every character's gender is swapped. A female version of Clyde is seen in a photo, alongside Lincoln's female version, Linka.
Girl Clyde has long black hair with a blue ribbon in it. She wears a yellow short-sleeved dress with a single, horizontal, white stripe on the middle. She also has glasses identical to Clyde's and purple hoop earrings similar to Leni's red ones. It can be inferred that Girl Clyde has a crush on Loki Loud, the male version of Lori, and has two moms (female versions of Howard and Harold).
This version of Clyde eventually ceases to exist, as it's revealed the entire ordeal is just Lincoln's dream.
Clyde is a fan of the Ace Savvy comic books, just like Lincoln. Lincoln and Clyde have cosplayed as Ace Savvy and his sidekick, One-Eyed Jack, respectively. In the episode "Sleuth or Consequences", the boys dress up as them for a comic convention that they ultimately never attend.
In the comic Deuces Wild! and the corresponding short, Lincoln and Clyde pretend to fight a giant gas monster as Ace and Jack. They are joined by the sisters' superhero alter-egos at the end of the comic to help defeat the monster.
In "White Hare", he appears as a beaver named Danny in Lincoln's dream.
Danny is a big brown beaver with a blue and yellow striped shirt. Danny is similar to Clyde in every way: awkward, caring, and nerdy, but sweet and supportive at the same time. He also seems to have an unhealthy and obsessive crush on Betty similar to Clyde's crush on Lori.
Like Girl Clyde, this version of Clyde also ceases to exist since it was just Lincoln's dream.
In "A Tale of Two Tables", it is revealed that Clyde knows much about table manners and likes helping his parents do the taxes. It is also revealed that Clyde has been at the grown-up table for his entire life.
As seen in the promotional short "Dinner Party", Clyde would probably sit between Lincoln and Lori at the Louds' feast table every Thanksgiving.
In "One Flu Over the Loud House", it is shown that Clyde's peanut allergy does not stop him from mixing peanuts into food that he prepares for other characters. From this, it can be assumed that it is not a life-threatening allergy, and that his parents can eat peanuts since they have them in their kitchen.
In "Out of the Picture", Clyde tells Coach Pacowski that he has had nine years of therapy with Dr. Lopez. If Clyde followed Dr. Lopez's therapy for a long time, who was also Howard's personal therapist, this means that he probably was adopted when was a very small baby (about 2 years old).
The official Instagram page for The Loud House occasionally misspells Clyde's surname as "Clyde McBryde" in their hashtags, such as on this video, and this image.
It is revealed in "Change of Heart" that, despite having a crush on Lori, he hates the way he acts around her, since he can't control himself when she's nearby.
It is revealed in "Tricked!" that Clyde nose bleeds when he's scared.
It is revealed in "What Wood Lincoln Do?" that he can't blow up balloons because he has an intense fear of them (which is called globophobia).
As revealed in "Absent Minded" that Clyde has a perfect school attendance.
In "Antiqued Off", it's revealed that he has a love for antiques, and also that he doesn't like magic shows (and only goes to them because Lincoln likes them).