Hello Italkian friend. I do know that all the answers above are quite reasonable but I'd like to teach you some slang about it.
In São Paulo we say that someone is your "Parça" That is a short for parceiro which means "partner"
We can also say that you guys are like "carne e unha" and that means literally, flesh and nail.
In some places in the northeast of Brazil you can say that you guys are like " a corda e a caçamba" which means literally the rope and the bucket.
I hope you can have a little bit of fun with those expressions and feel curious enough to visit Brazil.