A DEPENDANT is the 'one kept' . (Dependent is the adjective)
Dependent is also the archaic word meaning 'hanging down', so maybe dependant was the noun version -someone who was hanging down
I think dependent may be the answer sought though.
Dependent has an archaic meaning of 'hanging down' - i.e. in suspense
One kept could be a kept person, a dependent person- a dependant- one who is dependent.