Mr. Krabs and Plankton were born the same day and were also life long friends.
Mr. Krabs was raised as a very poor boy.
Patrick and Gary being related.
Mr. Krabs left one of his army friends inside a locker, forgot about him and pushed off his skeleton like it was no big deal.
Davy Jones runs Davy Jones' Locker.
SpongeBob for married in MM&BB. (Okay this one wasn't serious. :sbx: )
Patrick and Sandy both have siblings. (Still if-y if Mr. Krabs has one, though we know he has three nephews.)
Bubble Buddy has been the first character to progress enough to where he got married and had a son during the show.
To name some that come to mind.