Yesterday I was looking for a Q word to express 'companion' for my John Locke citation translation. I ended up using the attested otorno but was not wholly satisfied as I was rather looking for something more gender neutral (would *oquen be a viable alternative?).
Looking at the etymologies of the applicable English words it seems that they either have a slightly 'legal' feel or have to do with sharing food/quarters. Fellow (originally meaning sg like 'someone who puts down money on behalf of someone or to join a venture') and partner (from partitio, someone sharing a division of wealth, inheritance etc.) fall into the former category; companion ('someone sharing bread', cf. the [folkish and dated] Hungarian kenyerespajtás lit. 'bread-mate'), mate ('someone sharing meal') and comrade ('someone sharing quarters', ultimately from camera 'chamber') fall into the latter.

From the attested Quenya vocabulary there is asambar(o)/armaro 'neighbour, someone living nearby' and artarindo/astarindo/astarmo 'bystander, witness' which could suggest similar formations to express 'companion'. [Astarmo in particular is very suggestive to having to do with asta 'division' from SAT- thus giving it an air similar to that of 'partner', see above, although it's probably just a coincidence and not intentional by Tolkien.] Then there are the comitative prefixes as-/o- which could be used (see my *oquen above, perhaps *olendo 'fellow traveler', or maybe even *osambaro, a calque on 'comrade'?).

Finally, companion 'someone sharing bread' reminded me of my realization some time ago what a genius Tolkien was when he called prominent women, ladies massánie 'bread-giver', given that the etymologies of English lord (OE hlafweard) and lady (OE hlææfdige) are 'bread-warden' and 'bread-kneader', respectively.