He is the grumpy next-door neighbor of the Loud Family and he seems to get along well with them. After "11 Louds a Leapin'", he and Lincoln became friends. Like the Louds, he also has a big family, but lives far from them, and lives alone.
He first appears in the episode "Chore and Peace", but his name wasn't revealed until "A Tattler's Tale". Not much is known about his past, but it is revealed that when he was a child, his hair was brown, and he still wore glasses.
In the episode "Lock 'n' Loud", it is revealed that he's allergic to walnuts.
Personality
Bud Grouse is the typical grumpy old man of the neighborhood.
He doesn't like to see objects from the other neighbors in his yard, so he decides to keep them in his house, and doesn't give them back. He can get upset by the Loud family's constant noise and messes.
He is usually unhelpful when the Louds ask him for a favor. He agrees to help only when he gets food in return like lasagna or muffins.
It appears that whenever the Louds (possibly anyone) involve him in a plan, he'd usually explain the plan out when something goes wrong, as seen in "Lock 'n' Loud", when he said next time if he was going to pretend to be robbed, the muffins should not have walnuts, and in "Garage Banned", he also accidentally said that he crashed into his own garage instead of the Louds' garage.
Despite his grouchiness, it was shown that in "11 Louds a Leapin'" that he can be sincere and kind to others and that his grumpiness might stem from jealousy of the Loud family, due to him not being able to visit his own family, which is just as big as the Loud family.
Appearance
Bud Grouse is a heavy old man. He doesn't have hair (except for the two hair strands that he has on his head), but he has a white mustache.
He wears a white shirt under a black waistcoat, a blue tie, brown pants, and dark brown shoes. He also has a golden pocket watch in his waistcoat's right pocket, and a pair of little glasses.
Before Mr. Grouse's name was revealed, he was simply referred to as "Neighbor".
His name is revealed to be Mr. Grouse, as shown in the credits for "A Tattler's Tale".
The word "grouse" means "complain" or "grumble", which matches with his personality, as seen in "11 Louds a Leapin'".
A running gag in the series is him leaning outside of his window, and saying "Nice (insert object here) Loud!"
It's revealed in "11 Louds a Leapin'", that Mr. Grouse also has a large family, just like the Louds.
He is also similar to Old Man Marley from the first Home Alone film, as they are both lonely, and they both live next-door to the main characters.
In "11 Louds a Leapin'", he calls Lincoln by his name for the first time, instead of the usual name Loud.
In "11 Louds a Leapin'", he owns a Fearsome Flyer 1000 when he was a young boy, which he gives it to Lincoln as a christmas gift, to make up for destroying his sled.
He gets a redesign in "A Tattler's Tale".
In "Chore and Peace" and "A Novel Idea", his eyebrows are black instead of white, his head is shaped differently, and has no single strand of hair hanging from his head. This would change in every episode he appears in afterwards.
What's interesting to note is that his golden pocket watch doesn't appear in the aforementioned episodes, but it gets added in "A Tattler's Tale".
Dub facts:
His Polish surname is "Lament".
His Latin American dub name is "Sr. Quejón".
However, in Tripped!, he was called by his original name.
His French dub name is "M. Grognard" (which means "grumbler").
Mr. Grouse's Hebrew dubber, Saar Badishi, also voices Howard and Flip.
