Which kingdom is the leftover kingdom which contains distantly related species that just don’t fit into any other kingdom?

The kingdom you are looking for is Protista.

Explanation: Protista is often referred to as a “catch-all” kingdom because it includes a diverse group of organisms that do not fit neatly into the other kingdoms like Animalia, Plantae, Fungi, Archaebacteria, or Eubacteria. Members of Protista can be unicellular or multicellular and have various characteristics, including plant-like and animal-like features.

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