Foster's Mansion.

He's an imaginary friend, he dresses very elegantly and even seems like a kind of servant despite not doing anything, he's a bit arrogant at times and usually spends his time giving orders to Frankie.

She is the granddaughter of the owner of the Foster Mansion, she works there and also stays in this same place, she is somewhat sarcastic and very kind to imaginary friends.

He is the boy who created Blue, who is his imaginary friend. Mac usually spends time in the mansion, plays with Blue and more. He is very kind and sometimes a little sarcastic.

He is Mac's imaginary friend, he is a little small, he is usually very arrogant and stressful sometimes but he is a very good friend.

He is an imaginary friend of six or seven meters, red in color, he is missing his right arm, he is very calm, kind although sometimes silly.

He is a somewhat robust imaginary friend but despite this he is very fearful and kind, and is usually nervous when he sees new people.

She's a kind of chicken, she can't say more than the word "coconut," she's sometimes rude, and she can lay eggs that have a surprise inside, from a chocolate bar to a flamethrower.

She is an elderly woman, owner of the foster home for imaginary friends and Frankie's grandmother.
She's the one who lets other people adopt an imaginary friend.
He is the one who created Mr. Rabbit... therefore Mr. Rabbit would be his imaginary friend.