To start with, Angelica is the protagonist of a tragic story.
Because at one time they weren't imaginary.
Chucky and his mom died in a car wreck, which is why his dad is such a neurotic mess.
During a trip to Paris, Chas Finster met a Japanese prostitute named Kira. He married her, and she moved to America to be with him.
Phil and Lil were aborted.
Angelica's hallucinations are a subconscious reaction to her own tragic past as well.
The only real baby was Dil.
The two of them grow up to be deeply damaged individuals in Rugrats All Grown Up.
Angelica's hallucinations take the form of older kids, to compensate for the passage of time.
Susie is Angelica's only friend, and wants to help her get better.
But in the end, Anjelica's addictions drove all her friends away, including Susie.