Someone needs to go on an adventure or otherwise leave their usual routine behind. However, whether it be because of a Secret Identity, the Masquerade, or whatever, nobody involved in that person's normal life can know that they're gone. Thus, the Substitute Self is needed: a robot double, a hologram, or just someone else in disguise to take a character's place.

Related to Identity Impersonator, which is a temporary form of this done specifically to protect the Secret Identity by having the substitute and the main character seen together. Sleeping Dummy is a version of this for sleeping, which can be much less intricate because it doesn't have to act like the person.

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Examples:

• This was inspired by reading about some examples of this happening in Animorphs, but specifics are needed.
• The second The Santa Clause includes a robot Santa to replace the regular Santa- the robot then turns evil.