Good question; science yet don't know anything about "What is Life".
Yes. Everything about life we discuss (whatever by a Newton, Descartes or Schrodinger), it is in a level of Sci-fi, or hardly alchemy. Anyone of us don't know actually what is it.
The best characteristic of life is, "we, the living-creatures, can sense . We have consciousness" .
But alas, anyone can judge its own consciousness. We can-not judge someone else or any-other object contains any consciousness or not. We can just guess other's consciousness from facial expressions (say cry) , behavior, polygraphs, physiological reactions (respiration, growth, ageing etc... ), complication, informational-content, reproduction, genetic-code and such.
(https://www.youtube.com/watch?v=evQsOFQju08 , Is Your Red The Same as My Red? by Vsauce)
(however a living object can lose consciousness for a while such as when we're chloroformed).
In the same-way, we just guess presence of life in other objects from the complication, chemical structure (carbohydrate, proteins, lipid, DNA etc), metabolic reaction etc. and same evolutionary origin . same to us. In strict-logic we can't tell a lovely flying bird contains life and is conscious that an alien-robot is nonliving, lacking consciousness. We can guess, not prove, yet.
In the same logic, all virus, viroids and prions (the organisms of "border zone") (including biggest viruses) could be (and often is) compared with living-organisms (like we're), due to their similar chemical structure, genetic codes, information content, reaction etc. with us, as-well plausibly same origin with us.
There exist too, causes to consider all virus, viroids and prions (including biggest ones). I can call you alive. I can call your one organ (say hand) alive, could call a cell alive. But what we could call 1 protein-molecule? Just like a concrete-mixing machine (known to be nonliving), the protein-molecule is similarly made up of atoms... and nothing else. There is no evidence for "vital-forces" also... the protein molecule operates its works just with electromagnetic forces, thermal-collisions etc. There is no "sign" of life . In the same-way, a virus, viroid or a prion is just a lump of molecule. And for a big virus? a small hut (nonliving) : big house :: small virus : big virus. A big virus would be similar to a big bottle of nucleic acid.
Any more-discussion would be completely opinion-based, but in my-opinion it is better to consider these borderline-organisms as alive, due to more valid-logic such as
They're similar with us in chemical structure, genetic code bla bla bla ...
They are plausibly same in origin with us.
Inert structures, like plant-seeds if contain life, then in same logic we can imagine life in virions etc.
We found different level of parasitism in living organisms, such as ATP-parasites (I forgot example and can't find right now), and consider them as alive. So why we'll not consider a "protoplasm-parasite" a living-organism?