One of the interesting things about "Fallout 4" is that it features companions, who can assist the protagonist as he ventures the ruined Commonwealth.
A player will likely find companions, either human or not, along the journey and they can help scavenge weapons and armor. There are 12 companions within the game, but only one can assist the player at a time.
What's great about "Fallout 4" companions is that they cannot die. They can also be told what to do; they can carry weapons, armors for the player and even wear them if instructed.
Basically, a companion is a helpful ally along the journey to the wasteland. They can also disagree or agree on the player's decision, so choosing the right one is important in keeping the game as interesting as how the player would want it to be.
Spending a lot of time with some of them would even make them like the player to the point that they can be romanced. Romancing some of them would grant an XP boost for a short period of time.
Each companion also has unique attributes and set of skills that can be valuable during gameplay.
For instance, Nick Valentine is the perfect companion when it comes to hacking computers and picking locks. Being a private investigator, Nick has the ability to interrogate, plus he can hack computer terminals too.
These companions don't pop up just randomly. They have to be recruited after completing specific quests. Though they may offer their services to the player, it doesn't guarantee that these companions will stay for long.
This is why it is important to keep them interested in the player by recognizing their likes and dislikes.
Fortunately, a Redditor posted a detailed guide of each of the companion's likes and dislikes. This guide should help players make sure that their companions will be offering their assistance for a long time.
IGN has more information on the rest of the companions.