I was looking for some ridiculous build and I guess I found something. The companion feat says “You gain companion character that acts independently from you”. Does it mean that they can cast banes, bones and it counts like another pc would cast it?
Yes. That is how the Companion feat works.
It’s a seperate character with its own initiative and everything. Can be pretty op, but usually an op companion results in an underpowered character.
So can i create my own personal army of demons using my companions as generals, whom use boon “summon creature”?
You could, provided that your GM allows it. It would spread your PC very thin, but you would crush most normal encounters with sheer action economy. Of course, since your GM is running the game they can just start planning encounters around this, and this is not quite the most completely busted PVP build in existence 
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