Why is my girlfriends brother so damn weird?

SO he is 15 and im 16, He acts shy around me and meek, He sometimes follows me around or walks with me and he is pretty much in my friend group with my buddies, he does not start conversations ever and he literally does whatever me or my buddies tell him to, He also acts shy around them and me as well, Even when me and my girlfriend got in a huge fight months ago and quit talking for a couple days he literally told me to "get over her" and was talking shit on his own sister and told me he would talk sense into her, Then he told me my girlfriend was pouting about me constantly, We made up and are loving again, BUt even when we "broke up" her brother legit sided with me, i dont know why he acts so nervous and shy around me, When i visit my girlfriend at home he is nervous, I took him to school one day because the bus missed him and my girlfriend was sick, he was awkward and shy the whole time, Why has he always acted so strange around me? He is a very confident and athletic guy but acts so weird.
Why is my girlfriends brother so damn weird?
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