In "Fed Up", it is revealed that Lynn Sr.'s astrological sign is Libra, which means he was born on or somewhere between September 23 and October 22.
In "Home of the Fave", it is revealed that, when Lynn Sr, was a kid, his father, Leonard, played favorites with him and his siblings. Despite this, however, Leonard would always take Lynn Sr. on summer vacation to Camp Mastodon, as revealed in "Camped!"
"Recipe for Disaster" revealed that, in middle school, Lynn Sr. learned cooking from his home-economics teacher, Mildred Scalise, who often collaborated with him and gave him some unique cooking tips. The two wrote down in every recipe they created in identical recipe books.
According to "House Music", he tried to play in a music band, but his bandmates kicked him out, because they did not like his cowbell playing.
According to "L is for Love" and The Loud House Movie, he met Rita when he was walking through the street and they fell in love at first sight. Since she was too shy to directly talk to him, she slipped love letters to him whenever he passed by, eventually asking him out on a date at Banger's & Mosh.
Lynn Sr. wanted to be a chef, but to keep his wife and children fed and well, he worked as an IT specialist. He eventually quit that job in "Job Insecurity" and began working at the Aloha Comrade, where he initially worked as a dish-washer (a job he personally hated) before being promoted to co-chef. Several months later, he finally achieved his dream of opening his own restaurant, Lynn's Table, in "Cooked!"
Lynn Sr. is stern, and often calls a halt to his kids' fights, but he loves and cares about them. He also loves his wife, and often acts playfully romantic towards her. He has been shown to have a childlike side at times, even sometimes taking part in his kids' antics, when it's all in good fun. He's also the more sensitive of the two parents, often necessitating his wife, and stepping in whenever he needs to pull himself together.
He is also shown to be quite neurotic; as he screams and faints at the sight, sound, and idea of spiders, as well as Lana's mud wrestling, horror movies, and Halloween.
He has a habit of prefixing words with "ding-dang" when frustrated.
Lynn Sr. wears a sea green sweater over a light green checkered collar and cuffs, brown slacks and shoes. He has a long pointy nose, brown eyebrows, and curly brown hair that is thinning on the top.
In "House Music", when he is playing the cowbell, he wears a yellow-striped shirt, and green pants with a rip that exposes his underwear. In his youth, he sported a mustache, though he seems to have shaved it by the time he met Rita. His hair was also straighter and parted in the middle.
Lynn Sr.'s entire face was never shown during the first season. His face was revealed along with Rita's face in the Season 2 premiere.
He believes singing while cooking improves his cooking, but it took him years to perfect his "cooking ditties". In addition, he sometimes sings while doing activities unrelated to cooking, such as checking the mail and changing Lily's diaper.
He has a tendency to lose consciousness when he is afraid. For instance, he spent most of Halloween unconscious due to his fear of the holiday, and almost delivered Lincoln but he passed out before he was able.
He is "sensitive" to raised voices, and if he would pick a superpower, it would be flight.
In the Polish dub, his name is Henryk, and he is the senior of Lana (Henryka) instead of Lynn Jr (Helena).
Sometimes fear renders him unable to speak coherently, but Rita understands him nonetheless.
The first dish he ever served someone else was linguini with clams, which he calls "Lynn-guini", and which is apparently famous among his family.
Up until Season 5, none of his relatives from his side of the family physically appeared in episodes and only been seen in photos or flashbacks.
His father makes his first physical appearance in "Camped!" where his first name is revealed to Leonard.
As a kid, he slightly resembled Lynn Jr. possibly explaining why she was named after him.
As a child, he and his father would routinely go to a camp called Camp Mastodon, which is where Lynn Sr. learned to run, swim, use a zipline, and use Morse code.
His side of the family is descended from Scotland.
He sometimes speaks in a British accent, which he learnt to do while taking a semester in England. Rita finds this accent cute.
However, he is not the only Nicktoons character with this distinction, with another such character being Mr. Turner of The Fairly OddParents who is played by Daran Norris in both animated and live-action media of the mentioned series.