Timmy is likely a figment of Mom's imagination/drug hallucination, as shown in the video A day with Mom. This theory is backed up by the fact that he Is never seen on camera and he never talks.
He probably has more siblings. A definite one is the constantly abused baby. A 'Samantha' is mentioned in one of the episodes, but it is unclear whether or not she is cannon.
- Timmy is gay, as he is seen watching "workout videos" (actually gay pornography) and his mother refers to him as "clearly gay, but we haven't had the talk." He even admits it to Helen Brownstein in Hall Monitor Helen.
- Timmy is hostile towards Grandpa, constantly throwing things at him, spites in his face, flashing him his genitals, repeatedly tries to give him flowers (which he is very allergic to), and generally causes trouble for him.
- Timmy also seems to be hostile towards many other characters too.
- He also has a small penis, according to his grandpa.