If I were to make a power for it, it might be something like this. Game balance is absolutely not guaranteed here, as nobody but me has looked at this.
Companion [-1]
Description: You and another being are inextricably tied. Whether it be a blood brother, a familiar, or something else, the two of you are utterly devoted to one another and your fates are intertwined.
Musts: You must define an NPC character or creature to be your companion. Take an aspect that reflects your tie with it.
Effects:
Ally. Create a character sheet for your companion. It has 2 refresh to spend and half the number of skill points that your character does.
We are as one. You are subject to compels on both the aspect that defines your relationship to your companion and, in some cases, to any aspects that your companion itself has. You may also choose to spend your own fate points on rolls and declarations made by your companion.
Level up [-1]. Your companion gains an additional two refresh to spend on powers and stunts. It's total refresh may not exceed half of yours.