It's a Scottish accent.

I'm not sure how it comes across in his culture--is he supposed to be Australian?--but in the US I think it comes across as a bit affected. By that I mean, when people use that word a lot, it's usually because they are a little bit "counterculture" or something like that. Almost like a modern-day hippie. It's supposed to convey that you view the person you are calling "brother" as someone who is an ally or friend. But in my opinion it is a little rude or manipulative to use it with everyone you meet, because you're telling a stranger or acquaintance they are like a brother to you--and how could you really feel that way, if you barely know them? So to me it is a little "peacocky," or false. I guess it depends on the person using it though. But in general I don't like it when it's used indiscriminately. But, on the other hand, you could call someone "brother" in an ironic way, as a kind of joke. I do this with my own teenage son. I guess it's like a little joke, almost like a term of endearment, in this particular case.