In Fallout 4, you are not forced to travel through the unfriendly post apocalyptic wastelands alone, despite the fact that the game offers such option and can even reward you for that (Lone Wanderer perk). Thirteen different characters can join the party of the main character. What's important, only one character can accompany you at once, the other allies will be waiting in locations assigned to them for when it's their turn.
You can have the same companion during almost the whole game or toggle them depending on your needs. Each of them has their own unique profits (you must unlock them first!), their pros and cons. For example, you should take a companion that is good in battles for a quest during which you will participate in a large battle, and for a quest during which you must investigate something, you should take an ally that will offer you help in conversations with NPCs.
You can give simple orders to your allies or count on them to take care of their own. You can also give them the items you have, reducing the load of your protagonist. It is important to have good relations with companions. You can improve the relations with companions by making choices they agree on or by performing actions that he will approve (for example, opening a lock or using chemical). Good relations unlock access to new perks related to companions. Each companion has one unique perk related to him which after unlocking remains active for as long as the companion travels with you.
You can have a romance with some companions (information about availability of romances can be found on the next page). The option to start a romance becomes available after you improve relations with the companion enough. Sadly, this mechanism works in both ways and a companion with whom your relations suffered might leave you.
Selected companions have their own, optional quests. Depending on the ally, such quest can become available once you meet with the companion for the first time or after you improve your relations enough. The description of companion quests can be found in our guide with game walkthrough.