Jester

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A jester is a headmate who makes others happy or cheers them up, often through being witty and playful and making jokes or otherwise being silly, but not exclusively in these ways.

This can be towards those in their own system, or those outside of it.

Related Terms
Delight and comedian also describe playful headmates.

A mood booster similarly cheers up others.

This may be considered a type of caretaker.

History
This term may have been coined independently on multiple occasions; both coiners are credited for completeness.