Origins

 Before becoming a servant on the First Evil he was exiled from priest hood for being responsible for the death of two young girls One of these girls was a showgirl. Although not being seen until the last 5 episodes he is believed to play a major part in the first's movement. This is believed becasue he ordered "bringers" to kill potential slayers  around the world. It is later found out that he/the first was the cause for the bombing of the Watchers Council HQ in England.

Sunnydale

In one of his first appearances "BTVS episode Dirty Girls" he easily defeats the combined power of Buffy,Spike,and Faith. During this he uses his thumb to painfully stick it in Xanders eye blinding him. In the last episode "Chosen" he was finally killed by Buffy using the Scythe.
Later in the same episode the first uses Caleb to annoy Buffy to keep her mind off their upcoming battle.

Powers and Skills

 Because his body is a vessel for the power of "The First Evil" he has super strength that easily surpasses slayers and vampires. This reason also makes him almost immune to all damage. Only an enraged Buffy was able to slice him in half. But Caleb has been known to punch through stone walls easily. Caleb also possess Telepathy,Aura-reading and the ability to track his opponents.