For me, there’s a big difference in being a friend and being a bro.
One of the biggest examples I can think of of that difference is:
A bro will stick by you through thick and thin, and will defend you at all costs simply because he is your bro
A true friend will have the maturity to call you out on your mistakes and, heck, even snitch on you when he genuinely feels that you are wrong. But despite that, he harbors no hate for you and remains your friend.
I have absolutely no doubt in my mind that Yoshino is Kotarou’s friend, and a great one he truly is. But a bro is not what I would classify him as.
As @DaBackpack said, Masato is an even bigger bro than Yoshino, and that I can agree with. Masato holds no opinion, and does not complain. Not even after Kyousuke fails and causes Rin to revert. But he still supports both Kyousuke and Riki until the end. Comparing that to Kengo who was vocal in his opinions but still remains a friend at the end. In the same manner, I do not consider Kengo a bro, but an important friend of all the Little Busters
