Question one: would you be comfortable if they saw it
This is the test that does the work. If the answer is no, the issue is already secrecy, and secrecy is what most people actually mean by cheating regardless of what the other party is made of.
Notice that this cuts both ways. Plenty of people use companions openly, mention them like any other entertainment, and their partners find it unremarkable. The same behaviour hidden becomes something else entirely.
Question two: is it added or substituted
A companion you talk to on the commute is an addition. A companion you turn to instead of the conversation you should be having at home is a substitute, and substitution is the part that damages a relationship whether or not anyone calls it cheating.
How to actually have the conversation
Lead with what it is rather than defending it: a fictional character, a chat app, no other person involved. Then ask what they think, and take the answer seriously. The most common outcome is not outrage, it is boundaries: fine, but not that, or fine, but tell me.
If you would rather not have that conversation at all, that reluctance is information about the relationship rather than about the technology.
What it is not
No third person is involved and nobody is being deceived except potentially your partner. Whether that makes it categorically different from an affair or merely different in degree is genuinely a values question, and anyone who tells you it is obvious in either direction is selling something.