Tyrael

Tyrael is first seen in-game in Act II after killing

Duriel in

The Seven Tombs quest.

He then appears as an permanent NPC in the

Pandemonium Fortress in Act IV where he will resurrect mercenaries for a fee.
He also starts quests

The Fallen Angel and

Terror's End.

Later on in Act V,

Tyrael appears in

Eve of Destruction where, after killing

Baal, he opens a portal that the player can take to end the game.