Why Ayu using "boku" ?

[Aga-chan]

ayu said that using "boku" seems like another person. Another reason would be that it's from a man's view, which I think applys to "Free & Easy". The melody is also an aspect, because "boku" and "kimi" are shorter.

By the way, some girls do use "boku" or even the rougher "ore". Whatever you read about male and female language, better don't take it as a fact that is like that and only like that. Just like some girls use "boku", some boys use "ne" quite often or even use "wa" (meaning the sentence ending particle, not the topic marker).