All There in the Manual: His last name, from the book "Not Just Cartoons: Nicktoons!" is Rama. (as in, Rock-o Rama. Get it?) This is not mentioned anywhere in the series (due to the staff not reaching a consensus, he was left without one proper during the show), similar to the protagonist of another show.
Dysfunctional Family: His adoptive family is...interesting, to say the least. His biological family isn't as bad, but it's implied that Heffer's biological father didn't want him, his real mom was killed and made into a car seat in Illinois, and his stepmom is a sweet cow who's married to a Jerk Ass.
Coming of Age: In the episode "Born to Spawn", Filburt is angsting about becoming an adult as he is forced to go to Kerplopitgoes Island (his birthplace) for a Rite of Passage. Which turns out to be a theme party in honor of The Seventies, where all the nerdy turtles become temporarily cool.
I Was Quite a Looker: Word of God says he used to be handsome and athletic, but a case of unrequited love sent him into a downward spiral, which entailed a stay at a mental institution and culminated in his present Nerd status.
Mistaken for Aliens: In the 'diary' episode, Heffer and Rocko find out that Filburt is an alien bend on taking over the world, but naturally it was just a misunderstanding from his novel called "Filburt's Secret Journal." or WAS IT?
Younger than They Look: He's 38 in "Future Schlock", but looks more like he's in his 80's or 90's. At the end of the episode, he looks like his normal self, suggesting he just hadn't been taking care of himself.
Demoted to Extra: He's literally been through this at his place of work. When we first see him in Conglomo, he has enough clout to hire and fire Rocko. Later on, he's reporting directly to Mr. Dupette, who is now his boss. And even later on, in "Teed Off", he's described as the guy who "checks the bottlecaps on all of the bottles," being bossed around by one of the generic lizard workers!
Jerk with a Heart of Gold: His reconciliation with his son Ralph earns him this, but only towards his son. Otherwise, he's a...
Jerk With A Heartof Jerk: For example, think he was being generous when he offered Rocko a job? Two seconds later, he blows that sky high when it's revealed he only did so because the job was 'Test Subject' and it would have involved Rocko being mutilated or even killed.
Unknown Rival: Rocko and friends are aware of what a jerk Ed can be, but they have no idea that Ed hates them so much that he has an entire team of scientists dedicated to disposing of them.