God in His grace gives everyone good things, even those who don't believe in Him. From what you say, your atheist friend is an example of this. She's been blessed with a kind personality, although she doesn't realize who gave it to her. Jesus said that God "causes his sun to rise on the evil and the good" (Matthew 5:45).
I regret your other friend isn't this way, because Christ certainly doesn't want us to be sour, critical or unpleasant toward others. Christians aren't perfect, nor do we claim to be, and sometimes we fall far short of what God wants us to be. Perhaps if you knew your "nasty" classmate's background you might discover why she is this way, but her behavior is still wrong. God's will is for us to become more like Christ, avoiding sin and loving others the way He did.
Don't, however, use this person's failings as an excuse to turn away from Jesus. We may be imperfect, but Christ is not, because He was God in human flesh. And because He was perfect, He alone could become the final and complete sacrifice for our sins. As you strengthen your body, also strengthen your soul by turning to Jesus Christ and committing your life to Him.
Then ask God to help both of your friends discover the joy that Christ promises to all who put their lives into His hands. When Christ lives in us, we "are filled with an inexpressible and glorious joy" (1 Peter 1:8).