A person who in their essence contributes nothing to the situation or engagement at hand.
This term is often used as a moniker for someone who lacks personality, or interpersonal skills. As a result this "exister" chooses not to participate in conversation.
An "exister" has a negative connotation.
Also used in the present tense as "existing"
This person simply exists in all senses of the word.
"Stop existing on my couch you exister."