The verb remain is used a few different ways. If two countries remain at peace, they are continuing in a peaceful state. If a front wall is all that remains of a fort, that wall is all that is left after the removal, loss, or destruction of the rest of the fort. If you remain at home, you are staying at your abode. To linger is to remain or stay on in a place longer than is usual or expected. The use of this verb implies reluctance to leave. If a few friends linger after a party, it means they continue to hang around for a while after everyone else has left. Linger can also suggest a musing or contemplative quality. If you linger over the beauty of a painting, for instance, you are dwelling in contemplation, thought, or enjoyment.
The noun friend is a broad term that suggests varying degrees of familiarity. It can refer to someone who is on good terms with another (Who goes there? Friend or foe?), to a supporter or patron of some institution (friends of the Boston Symphony), or to a connection on social media. Most commonly, though, a friend is a person with whom one is on intimate terms and for whom one feels a warm affection. This is the meaning that gets us closest to the noun companion. A companion is a person who shares one's activities, fate, or condition. You might have a traveling companion or a companion in despair. The key difference is that a companion is a person who you are frequently in the company of. A friend may be far away, but still hold a special place in your heart.
The standby adverb always is used a few different ways. It can be applied to things that happen on every occasion or without exception (he always works on Saturday), to things that occur continuously or without interruption (there’s always some pollution in the air), or it can mean “forever,” as in, Will you always love me? The adverb invariably is a synonym for always when talking about things that happen every time, without exception. If the weather forecast is invariably wrong, then it is never accurate (and maybe it's time to find a new source for your weather info!). If an answer to a question is invariably “yes,” then it is affirmative every time the question is asked.