Rei Kinoshita

Rei is always met with bad luck. It turns out that she is an Atarunian; an alien that is forever plagued with bad luck. She comes to earth looking for a special sphere-shaped rock she had lost during her last visit to the planet. The rock balances out her incredible bad luck with good luck. She finds Fuyuki who had found the rock when he was a little kid and befriends him in order to retrieve the rock. She wipes his memory of her when she leaves. Momoka hated her because she thought she was trying to win Fuyuki's love but as everyone knows, Momoka would do everything to prevent that.

(Source: Keroro Wiki)