Theodore is an old man who temporarily rented Sheldon's old room in Apartment 4A.
In "The Property Division Collision", Theodore, the creepy old man that Sheldon rents his old bedroom to so he can get back at Leonard. Theodore also offers an analysis of their fight actually giving them good advice. He makes Sheldon and Leonard realize that there is simply a result that they care about each other and that it is hurting them to leave each other.
He is never seen again after this, since it is revealed that as of "The Emotion Detection Automation" (S10E14), Theodore no longer lives in Apartment 4A anymore since Sheldon's old room was becoming a game room and Penny's brother Randall would be coming and in "The Escape Hatch Identification" (S10E18), Theodore's departure is once again confirmed when Raj moves into Apartment 4A into Sheldon's old room.
It is possible he is now in a home for elderly/retired people or another type of home.
Theodore is seen as creepy, given the subjects he talks about. But he is also seen to be helpful as he helps Sheldon and Leonard realize that it's difficult for them to be living apart from each other.
- There was no applause for Theodore's first appearance, which should've happened due to him being played by Christopher Lloyd, a very famous actor.
- Theodore likes the Amy's painting of her and Penny and thinks that it brings the living room together.
- His blood type is O negative.