It remains to be known what caused imaginary friends to exist (aside from a kid imaging them and what caused kids to be able to imagine them).
Imaginary friends are created through the imagination of a child who desires a friend. Imaginary friends can come in all shapes, sizes, species, etc. and their creation and form don't seem to be limited by anything so long as the child can think it, it can happen. It seems as though only humans can create them as imaginary friends don't seem to be able to create imaginary friends. The fact that imaginary friend creation is not as simple as it seems is further proven when Bloo goes to Mac's school's show and tell and impresses the kids so much that they try to imagine an imaginary friend like Bloo. However, none of the imaginary friends match Bloo in appearance or personality. This could be because it seemed most of the imaginary friend take some sort of physical trait based off their creator's own appearance or personality. it could also be because imagining an already imaginary friend is difficult or impossible or that the process of imagining an imaginary friend is simply more of an occurrence that just happens rather than something that can be controlled.
They can be humanoid, animal like, anthromorphic objects, and seem limited in shape and size only by the imagination of their creators. Most imaginary friends are nude.
Due to the fantastical nature of imaginary friends, their biology vastly differs from humans. They can be an object or food or anthropomorphic or something from a fantasy book. Nonetheless, most of them have to eat. In "dinner is Swerved" the giant turkey leg mentions he doesn't eat as it would be killing his own kind but in "Read 'Em and Weep" a food imaginary friend called Sam Burger mentions that he is hungry and had to eat coconuts when stranded on an island to survive. When Eduardo was hungry in "Let the hare down" he ate chairs for three weeks, showing that imaginary friends stomachs can digest objects that humans would die trying to eat. In addition, imaginary friends seem more flexible and because of their cartoon like bodies can be stretched and stuff into places or endure far more pain than most humans. However, they are capable of dying.
There is little understanding about whether an imaginary friend ages or not, but it would appear that they can age but possibly do so slower than humans.
It is implied imaginary friends cannot reproduce.
Most imaginary friends have a variety of personalities and individual freedom, though this depends. Some imaginary friends are anthropomorphic objects, animals, people and even food. Interestingly, all imaginary friends seem to have intelligence. There are a few imaginary friends are extremely dangerous and cruel. They are called extremosauruses and are locked away as they will eat kill and/or destroy anything in their path.
Relationship with creators
Creators can only be children. The ability for one to create imaginary friends is lost upon completion of puberty (with the oldest shown creator of an imaginary friend being Mac's jerk brother Terrence). Imaginary friends seem to be mostly companions for children when they are young and it is the trend to abandon an imaginary friend and put them up for adoption once a child is considered too old to own one. Despite this, there are a few imaginary friends who remain lifelong friends with their creators, the most notable examples being Mac and Bloo and Ms. Foster and Mr. Herriman.
Relationship with family
Friends seem to have names bestowed upon them by their creators but never seem to have a last name or take on the family name of their owners (possibly since their owners will one day abandon them). While close to their creators, they don't treat their creators as parents but instead as friends and act as their companions. Their relationship with the parents is mostly varied, with some forcing a child to abandon the friend (like Mac's mother or Bendy's creators' parents) due to them being too old, not being able to afford the friend, or because the friend causes too much trouble for them. Parents are usually the ones who control whether or not a child is allowed to have an imaginary friend.
While it remains to be known how long imaginary friends live, they don't seem to age (or at least not at the same span as humans). Wilt looks the same as he did in the past. Nonetheless, it is possible to injure or even kill imaginary friends. It may be possible that they can die of old age considering how easy it is to make imaginary friends.
Interestingly, the process of creating an imaginary friend can be simple but difficult for some people. Terrence had a hard time imagining a friend at first but did manage to create Red when he relaxed and thought about the traits he wanted Red to have. In addition, the spoiled rich girl in the debut episode could not imagine one, claiming she shouldn't have to waste time imagining one instead of simply buying one (her mother states it is because she gets headaches). It seems that creating an imaginary friend requires an imaginative mind, allowing for people with high imagination like Goo to easily make as many friends as possible. However, it is also possible to show self restraint in imagining a friend, as Goo has shown in subsequent episodes.
Interestingly, despite the fact children can imagine imaginary friends, many families choose to adopt one. This seems strange. This could be because only certain children can produce imaingary friends or children don't wish to imagine imaginary friends that prove to be undesirable and get rid of them. It could also be that the process of creating an imaginary friend is a random occurrence more than something that can be controlled so children don't wish to imagine a random friend and would rather adopt one.
Traditionally, imaginary friends who wish to be adopted go to care facilities specialized for them such as Foster's home for imaginary friends and be cared for until some child adopts them. Interestingly, despite the fact children can imagine friends, many choose to instead adopt one.
Possibility of relationships with humans
While imaginary friends have been in relationships with each other, the idea of an imaginary friend and a human being together has never been addressed. However, in "Frankie my dear" Prince charming (an imaginary friend) showed romantic interest in Frankie, so the idea doesn't seem entirely implausible but is not addressed in the story.
Basic Purpose of Imaginary friend
The basic purpose of an imaginary friend seems to be to provide companionship to their child creators or serve some basic purpose the creator had in mind. Sometimes, imaginary friends like Dorkface or the Abraham Lincoln pen are created to do their creator's homework. Sometimes, teenage jerks create extremosaurus's because they want to see them fight and be destructive Some, like Ivan, were created to help their child (Ivan's creator Stevie is blind so Ivan was imagined to be his eyes).
True Purpose of imaginary friends
The main purpose of imaginary friends seems to be to act as companions and/or servants to their creators. However, there seems to be another purpose. While imaginary friends are usually created based on the creator's desires and demands at the specific time of creation, the creators themselves don't always have control over what the personalities of their imaginary friends are. Sometimes the imaginary friends strongly resemble their creators as shown in Good Wilt Hunting, but they seem to also represent something their creator lacks. For example, Wilt was created by Jordan Michael for the purpose of teaching him to play basketball, though in reality he had been created to teach Jordan how to be a better person. In Eduardo's case, his creator Nina created him to scare away the neighborhood bullies and crooks but be kind and be able to help take care of her baby brother along with someone she could play with. Nina also states that he seems to have been created from her subconscious in order to help teach her how to defend herself and others, a lesson she didn't realize until she got older.
This seems to support the relationship between Mac and Bloo and Mr. Herriman and Madame foster. The imaginary friends have nothing in common with their creators but retain a close relationship. It is possible the reason Bloo is selfish, narcissistic, rude, and arrogant is because he represents Mac's need to become someone who has more self confidence and be more adventurous while Mr. Herriman may represent Madame Foster's need for order and more strict behavior. The relationship between Red and Terrence seems to symbolize this as well, as Terrence wanted to imagine a friend big scary and ruthless who would destroy Blood but red turned out to be sweet and friendly, only turning violent when provoked. He could represent Terrence's need for compassion, patience, and respect (none of which he has or even seems to want as he rejects Red when he shows these traits).
Treatment of/Caring for imaginary friends
Most imaginary friends, while acting as companions, receive a wide variety of different treatment depending on who owns them. While poor treatment of imaginary friends is condoned, it is unfortunately not uncommon. One of the biggest examples being in the episode "The Stench of Success" where Bloo is tricked into signing adoption papers with a sleazy director who treated Bloo like a slave and went so far as to lock him in a cage and force him to work long hours of commercials with little room for sleep. In "emancipation Proclamation" the greedy Abraham Lincoln pen imaginary friend sells off all of the imaginary friends to be slaves, further proving imaginary friends due occasionally suffer maltreatment.
Certain imaginary friends are in fact servants, the best example being Scribbles.
Another example would be in "Room with a feud" where a calculator imaginary friend is adopted by who seems to be a jock/bully who commands his new friend to do his homework for him.
Some are unfortunately imagined just to be food and eaten. The main examples being in "Food is Swerved" where it is shown that there are food imaginary friends that go into hiding for fear of being eaten (a perfectly justifiable reason). Also, in "Seeing Red" Terrence is hungry and inadvertently imagines a pizza imaginary friend whom he eats with no regard.
This seems to be largely due to the fact that imaginary friends are not considered people and therefore seem to suffer some maltreatment as a result (especially those that are objects faced, imagined as servants, or are supposed to be food supply).
Mr. Herriman once mentions there being an imaginary friend dietician exists in World wide Wabbit, implying imaginary friends have slightly different diet or their biology is so different they have their own doctors, physicians, etc.
Life after abandonment
When a creator gets old enough or when a parent chooses that they no longer wish their child to have an imaginary friend, the friend is abandoned. The exact measures taken or what becomes of an imaginary friend afterwards is unknown. It seems imaginary friends can receive jobs as Coco managed to find a job in "Store wars" and Mr. Herriman did as well in "Setting a President". While Foster's Home for Imaginary Friends does serve as a care facility, no other facility has been shown so far.
Imaginary friends, though normally quite harmless, can be imagined as being powerful beings, the most well known example being World, an imaginary friend with what appeared to be unlimited powers in its own dimension.
Though it is unknown how many friends there are in the world, due to the ability of children to imagine an unlimited number of imaginary friends, it is possible they make up a large portion of the population.