Companions are a great resource for you to take advantage of in Fallout 4. Those are characters who can decide to help you based on the way you deal with them: if they love you, they will help you as well, so it's up to you to deal with them properly. You can see below a list of companions you will meet throughout the game, with their specific properties, pros included, and the locations where you will find them.
Dogmeat (Dog)
You'll find Dogmeat shortly after exiting Vault 111 as you head back towards Sanctuary Hills.
Codsworth (Mister Handy Robot)
Codsworth is your Mister Handy that has been waiting for your return for centuries.
Piper (Human Woman)
Piper is found in Diamond City. In fact, she's the young woman that helps you enter the city.
Nick Valentine (Synth Man)
This private eye is found in Vault 114 as part of the main quest Unlikely Valentine.
Paladin Danse (Synth Man)
This Brotherhood of Steel member can be recruited after helping to clear the Cambridge Police Station in Boston (not too far from Diamond City).
Deacon (Human Man)
Deacon can be recruited once you make your way to the Railroad HQ in the Old North Church. He is a master of stealth and disguise
Cait (Human Woman)
Head over to the Combat Zone in Boston. Recruit Cait there, she's a savage fighter and good at picking locks.
John Hancock (Ghoul Man)
He's in Goodneighbor where he hangs out near Scollay Square.
MacCready (Human Man)
He is in the seedy town of Goodneighbor. Completing the quest Long Road Ahead will let you bring him along during your journey.
Strong (Super Mutant)
During the quest Curtain Call, players will be able to free this Shakespeare-loving super mutant from a cell at the top of Trinity Tower.
Preston Garvey (Human Man)
Preston, leader in the Minutemen army, is found in Concord during the quest When Freedom Calls early on in the game.
Curie (Mrs. Handy Robot)
This companion can be found in Vault 81. She can even become your girlfriend.
X6-88 (Synth Man)
X6-88 is obtained shortly after making your way to the Institute as part of the main quest line in Fallout 4. He is excellent at long range.