Caleb

Buffy "We've got a new player in town. Dresses like a preacher. Calls himself Caleb. Looks like he's working for the First..." (7.18 - Dirty Girls)

Why The Vessel?

Caleb "And you're in me. Give me strength no man can have."
Buffy/First "And the only man strong enough to be my VESSEL, and I know you feel me, but I know why they grab at each other. To feel. I want to feel. I want to wrap my hands around some innocent neck and feel it crack..." (7.20 - Touched)

Quotes and caps

7.18 - Dirty Girls: Quotes - Screencaps
7.19 - Empty Places: Quotes - Screencaps
7.20 - Touched: Quotes - Screencaps
7.21 - End of Days: Quotes - Screencaps
7.22 - Chosen: Quotes - Screencaps

Credit

Quotes from Buffyworld.com
Screencaps from Nathan Fillion Italia