Suggestion has this line:
If you or any of your companions damage the target, the spell ends.
What counts as a companion?
This might sound silly, but I'm worried if a friendly NPC who tags along does count as a companion. Part of it is because the NPC is played by DM, and thus might ruin any charm based spell.
Is the definition decided by the target? Can I hide the fact that a character is in our party?
We walk as a group, with another ranger hiding in a separate location (thus we do not look like a group). If the ranger damages the target under Suggestion, does it fail?
Do NPCs fighting alongside us count as companions?
The king sends two guards to guard us. In response to an ambush, they (unknowingly it's been Suggested) attacked the target.
In a chaotic situation, can a target of Suggestion misidentify someone as my companion?
A tavern brawl. Party members involved in the "party". I've suggested a dwarf to go back to his home, then he got hit by another patron. This patron have casually talked with us, and seen by this dwarf before the brawl. Does Suggestion end because this patron (which is not our companion) hit the dwarf?
Best Answer
"Companion" isn't a game term, so it uses its standard English definition
In the context of 5e, "companion" doesn't have any exact rule or definition. With that in mind, Merriam-Webster defines a companion (in this context) as:
So, "companion" refers to anybody who is traveling with you. In a fight, it's your friends; or the people with whom you are allied. That is, the people on your side. If the friendly NPC is traveling with you (keeping company with you) then they are your companion and they could break the charm. Yes, that might give you one more obstacle to work around, but it shouldn't be any more of a hindrance than if you had another PC in the party.
You could try to keep the relationship a secret
Just like in the real world, you can hide your relationships with other people. Secret affairs and interactions are not unusual. However, just because I try to convince somebody that I don't know my best friend doesn't change the fact that we are actually friends in reality.
So even though you can try to lie to the people around you and claim that you are not friends or a companion of somebody else, the DM is the one who gets to make the final call. And if they think you two are companions, then you are companions as far as the game is concerned.
If the DM leverages that to ruin every charm spell you use, then you need to have a conversation
A friendly NPC is just that; friendly. Just like any PC, unless they have some deep character motivation to act against your use of charming spells, they would do their best not to disrupt your spell. Especially if they are in on the plan to suggest/charm a given target.
By that reasoning, if the DM is using NPCs to ruin your charm spells just for the sake of keeping your charm spells from working, then that DM is being adversarial. In that case, it might be worth talking to them about why they won't let you use these kinds of spells.