In Elex, you can travel with one companion. They are great help during fights and the entire experience for killed enemies goes straight to you. A companion can follow you almost everywhere (they have their own jet pack which means that they can get on top of mountains or rooftops of buildings without a problem).
You must remember that each companion has their own character and they usually belong to a faction – when you speak with an NPC you can find that some dialog lines are blocked because of your companion; they also participate in conversations. Usually, in order to get a companion, you must complete their quest. However, there are those, that can join you right away. More information about companions can be found in Lists chapter, Companions page.