Castillon

Castillon is a powerful Antivan merchant with a great deal of legitimate and illegitimate ties in Dragon Age II.

Background
The employer of Isabela and Hayder, he paid Isabela to escort a ship of slaves and a relic for him. The slaves were Ferelden refugees fleeing the Blight that paid Castillon for transport. He took their money and in turn betrayed their trust that was put in him. When Isabela frees the slaves, he tasks her to retrieve the relic and Hayder to track her progress.

Involvement
In order to meet Castillon, one must first convince Isabella near the end of Act 2 in order to have her come back (with the Qunari Tome as well) in the final battle with the Arishok. Only by doing so will the player unlock Isabella's final Companion quest.

Result
In Act 3 during the quest "No Rest for the Wicked", the player can choose to forcefully find and kill Castillon, thus gaining Rivalry points from Isabella, or allow Isabella to willingly turn herself in and have the player follow her trail, thus having more chances of actually meeting Castillon. You can also retreive incriminating documents about Castillon from the first part of the quest in The Blooming Rose brothel. If the latter option is picked, then Isabella will threaten to reveal Castillon's slavery ring plans to the Kirkwall authorities, but the player can make Isabella choose to give the documents back to Castillon on the condition that he will leave his ship into her command and swear never to hunt her down again.

If you bring Aveline or Fenris along during this quest and choose the option to make Isabella give the documents to while letting him go, you will receive +10 Rivalry points from both of them.

Note
I killed Castillon against Isabella's wishes (maxed out "Friend" so no penalty) and wiped out his crew. Strange thing is that his corpse wasn't lootable. Not sure if this is a bug. Figured he'd have a nice rogue knife or something.

I killed Castillon assuming the same thing, that he would leave something good behind to pick up. Thing is I saved it as my character stood over his corpse and reloaded a previous save so that I could pick a different option than killing him just to see what happens. Turns out I liked the idea of killing him more than not so I reloaded my saved game with my character standing right by a dead Castillon. When the game loaded I found that Castillon was glittering as though he had some loot to pick up but still I was unable to do so. Gotta be a bug at present.