Harry scared the children because they were afraid of "cooties" which also caused the girls to avoid him. However, Mojo Jojo noticed this and used Harry as a way to scare the PPG from crime. In the end, Harry ends up kissing Blossom and then the girls all kissed him all over his face.
Although he was used for evil intentions, Harry himself didn't appear to be evil at all, rather just a boy looking for someone to return affection to him and to not be afraid. Also, Harry didn't appear to be terrorizing off the Girls intentionally; he didn't even have to try nor did he have to say anything at all, be it menacing or reasoning. One could say that Harry was a living kryptonite to the Girls before they realized cooties were not real if he could defeat them so easily with unintentionally scaring them.
- His name is a pun on the term "hairy pits".