Frog is a frog who is the protector of Queen Leene.  He is first met in the cathederal in 600 AD, saving Crono and Lucca from an ambush.  At this point he joins the group.


Spoiler alert

Frog started out as a human named Glenn, who befriends Cyrus after he saves him from some bullies.  Cyrus tells him to get tougher.  Later, when Cyrus announces his intention to join the knights, he asks Frog to join the knights with him, but he refuses, claiming he would be too scared.  When Cyrus and Frog leave to go get the Hero Badge, they succeed, but encounter Magus and his generals, Ozzie, Slash and Flea.  Cyrus sacrifices himself to let Frog escape, but he doesn't.  Instead, Magus casts a spell on him turning him into the form that he is known for.