Leo is the Charmed One's whitelighter, which is a guardian angel. He has the power to heal people, orb, and to be able to sense his charges and/or suprnatural beings. He fell in love with Piper, and is now married to her and will have a child soon.
Cole is an ex-demon, called Belthazor. Later on in the series, his demon half was vanquished, but he gained a new demonic status. The Source of all evil. That too was vanquished, and now hes aquired even more powers, but is trying to use them for good. He loves Phoebe, and is married to her right now.