When someone sits, walks, or talks companionably, they are with you in a warm, easy way — not stiff, not showing off, just pleasant to have beside you. It is an adverb: it tells you how a person is being in that moment, not what they are. You meet it more in stories and careful writing than in everyday chat; a friend would more often say “we had a nice time together” than “we sat companionably.” It likes quiet shared things: sitting on a bench, eating without hurry, reading in the same room. Though actually it does not mean the other person is your partner or official companion; it only paints the feeling of being easy company.