Crono: Gluttony
Crono reminds me of Goku from DBZ. They both like a good time, both like to fight. And they both are essentially good. However they both eat like pigs.
Crono, first, drank Ayla into the prehistoric ground. He may have eaten an old man's lunch. If you're successful, you can fill a table with empty "soda" cans at the fair. He freeloads at the castle quite a bit. In the future he "sponges" off Marle's dad, which probably includes more food freeloading. They try to get back in good graces with the king, and so they try to give him jerky. After he dies, he is revived with an Egg.
Magus: Wrath
I'm sure Magus wakes up everyday "If a demonic outer space tick smites you on one cheek, smash him on the other." And we can't forget his most badass line ever. "I abolish all those who block my way, without exception."
Marle: Sloth
She's royalty. She shirks duty in order and her life as a princess to go to the fair. I don't blame her for it. But it makes her guilty of Sloth!
Ayla: Lust
I seem to recall her saying she'd go for anybody as long as they're strong. No doubt, strong loving to ensue.
Frog: Envy
I debated making him Wrath, but there's a decided sad note in how he lives. Cut off from human society. He envied Cyrus. His envy was indulged somewhat when he got the Masamune. Notice, thats the brightest point personality wise in the game for him aside from the Hero's Grave Quest.
Lucca: Pride
No need to explain that one really.
Robo: Greed
Robo is probably the hardest to hit. But I say Greed. He's greedy for friends and purpose.
The hard part about all of this is thinking of them all as sins, or bad things. The characters may be guilty of said things but they're not exactly malevolent (well, not all of them).