List of characters that appear in Foster's Home for Imaginary Friends.
- Bloo- Mac's imaginary friend, best friend and main antagonist of the series.
- Mac - A bright and somewhat shy 9-year-old boy who visits Foster's.
- Frankie - Madame Foster's 22-year-old granddaughter, addressed as "Miss Frances" by Mr. Herriman. Frankie is the caregiver at Foster's and helps keep everything in order.
- Mr. Herriman – A British-accented, man-sized, bipedal rabbit friend imagined by Madame Foster.
- Wilt – A very friendly, very tall, nerd and red-colored friend with a stubby left arm and crooked left eyestalk (which he describes as "wonky").
- Eduardo – A Spanish guardian friend. Despite his large size, overwhelming strength, and menacing demeanor, Eduardo is quite docile, timid, and compassionate.
- Coco – A bird-airplane-palm tree friend who can only say (or write) her name at various speeds and different emphases.
- Madame Foster – The founder of Foster's. Madame Foster is elderly but has child-like boundless energy, and enjoys life to the fullest/crime.
- Cheese – A simple, pale-yellow-colored friend who debuted in "Mac Daddy". Cheese was thought to be an imaginary friend accidentally created by Mac, but was actually created by Mac's neighbor Louise. He appears to be somewhat madcap and dim-witted, often saying incoherent or non-sequitur phrases, and breaking into sudden bouts of screaming when frightened or when he doesn't get his way. Cheese likes goldfish crackers, cereal, juice, chocolate milk (although he is lactose-intolerant), and so on. His name can be paired off with other characters' names to make puns-of-sorts ("Mac and Cheese"/ Macaroni and Cheese,"Bloo Cheese"/blue cheese, "Cheese and Crackers," and "Cheese Louise"/ the expression "Geez Louise!"). In the series finale "Goodbye to Bloo", he becomes the newest resident at Foster's, much to the residents' surprise/horror. Cheese is always considered to be an annoyance, but is treated like a friend by Mac's friends in "Infernal Slumber". He is voiced by Candi Milo.
- Duchess – A "high-maintenance" pedigree friend who is pompous, ugly, rude, and lazy up to the point she orders Frankie to open her eyelids for her first thing in the morning. She considers herself superior to all the other imaginary friends, is extremely negative towards everyone, and never says anything nice (despite this, she has helped someone in four episodes but only to help herself). Whenever Duchess turns on the spot, her entire body pivots like a sheet of paper being flipped, revealing that she is two-dimensional (however, in the pilot movie "House of Bloos", she was not two-dimensional). Her full name is "Her Royal Duchess Diamond Persnickity the First, Last, and Only". She is voiced by Grey Delisle.
- Goo – A hyper-imaginative, hyper-talkative 10-year-old girl who first appeared in the episode "Go Goo Go". Her parents allowed her to name herself when she was a baby resulting in the full name "Goo Goo Ga Ga". Goo enjoys playing games such as Checkers and Truth or Dare, but she does not know how to play and Mac is the only one who notices. In her first appearance she constantly created friends and finally stopped at the end. However, she has still created a few by mistake or to help on rare occasions. Both she and Terrence had a crush on Frankie. She is voiced by Grey DeLisle.
- Terrence – Mac's 13-year-old brother. He enjoys keeping Mac, and coming up with various schemes to make Mac's life miserable. His stupidity always gets the better of him, making him easily outsmarted. He is voiced by Tara Strong.
- Berry - A sweet, short, red imaginary friend who is in love with Bloo, which causes her craziness for him to ensue, even to the point of trying to kill Mac so Bloo would no longer be with him and would be with her instead. Though she only appears in two episodes, she is still considered a minor antagonist. She first appears in "Berry Scary". She is nice at first, then she fell in love with Bloo in first sight, at which point she reveals a "sour", borderline psychotic side to her. She meets Mac, but wants him out of the picture so Bloo will be hers. Bloo doesn't return the feelings and can't even remember her name, but this doesn't stop Berry from believing that they are "meant to be". She returns in Season 5 in the episode "Affair Weather Friends", but does not reveal herself until the end. When her plot to kill Mac by running over him with a toy train fails, she winds up getting trapped in a giant rubber band ball, as she did in "Berry Scary". She is not seen or mentioned again after that. The combination of Bloo and her name makes the word, "Blooberry", a parody of the word "Blueberry" She is voiced by Grey Delisle.
- Old and Young man Rivers -