A basic companion character is a basic monster stat block for the pc's level. The book recomends 1 basic power and 1 encounter, (2 at 11th level iirc). I played with a few of these as dm and they do fill a role in making it easier to have more characters and not take anything from the players and their characters. They are intended to hand to a player for combat and not overburden the player.
I tend to give these that stay around for a few levels a boost in power to make them a bit more cool, although not in league as the pc characters. At 7th level I give another encounter power, or I'll change another power to boost it a bit. At 11th level I gave one a recharge power for his only cool attack. I gave a few a magic item or two, but they do not count towards attack and defence. A magic weapon so he can throw it and it returns. A necklace that gives protection 5 fire, and such.
I also find that I can have regular fights without scalling back because these characters only have 1/2 or 3/4 the power as a regular character. It seems the other players in my group have more power gamer in them and can cary the slack.
I know they give a guide depending on the role of the character, controller, artillery, etc. Attacks are 4+level, slightly below MM3, but you add weapon proficincy. You do not get magic or armor bonus, but you get +1 each level. It all mostly works out. The biggest point is that it is easy and doesn't use too much brain power when you play.
This could be something 5E uses. Where one character is very easy to set up and play, but still works and is fun.