WTH would you want Kratos instead of Zelos? This is how you do it:

Go to every single glowing cricle in Tethe'alla and Sylvarant, be mean to everybody except Kratos, bring Kratos to fight Kvar and do everything Kratos-Friendly. Then (SPOILER)Zelos will betray you at Dirk's house and you have Kratos.

Personally, I was nice to Zelos, Raine, and Regal, but you can be nice to three characters, and build Affection Points by being positive in optional glowing circle skits and

Proto, you must be referring to your own path, because any three characters can knock on Lloyd's door, even if Kratos and Zelos both like Lloyd, if Kratos likes him more, you'll get him. Really, it dosen't matter who you're nice to otherwise, to get Kratos, be nice to him more than anybody else. You can probably get a huge FAQ somewhere.

Regarding GRADE, just fight a lot of enemies but avoid getting hit and use a lot of Tech Points.