Older Brothers

Side-character Onii-chans are inherrently cool because they are RESPONSIBLE, which is something usually the protagonists lack.

Similarly, because the protagonists typically lack RESPONSIBILITY, protagonists that are onii-chans are usually TERRIBLE PEOPLE.

I guess it could be similarly stated that imotos that are side characters are usually innocent and naive about the world while imotos that are protagonists are more aware and want to be more prominent and responsible (like their onii-chan!), but in this case both are still moe.

ACTUALLY I think in general there is just a terrible amount of typical male protagonists out there. Sigh.