And stand 'gainst the darkness.
Cyrus is Frog's best friend. He was probably some type of squire for Cyrus as well. Frog continues living on for Cyrus. I'd say these two are pretty close, like brothers. Frog gets bullied as a child as Cyrus tells him 'he needs to grit his teeth, but he replies 'it hurts when they hit me' to which Cyrus calls him a 'marshmallow'. Way to be supportive Cyrus. But as Frog grows up he is always with Cyrus and Cyrus convinces him to join the Knights. (Similar to Zack and Cloud of Final Fantasy VII.) Then after Cyrus dies, Frog goes on a mission to avenge him.
Queen Leene is her royal majesty who fully supports her Knights. She is a very important figure to the kingdom. Frog wants to serve her as Cyrus did before him but he feels unworthy. So she waits until he is ready. I think Frog is one of those Knights who loves his queen but is not inlove with her. Later on, Frog does become one of her Knights. He just want to be an honorable knight like Cyrus was for his queen.
Magus is the arch nemesis of Frog. He killed Cyrus and turn Frog into a frog. While both men can be recruited they will not have any interaction with each other and you don't have to worry about one leaving if the other joins. Though, there is a point in time before Magus is recruited you can chose to fight him but it'll be a one on one between Magus and Frog. (I opt out of this fight because if you fight him, you don't get him.) They hate each other but they will tolerate each other as well. Frog also hates Magus's henchmen Ozzie, Flea and Slash.