Andrei:

Defeat Andrei in level 1, and do not brainjack Goulnaba in level 2.

Badelaire:

Go left on the train tracks in the Lyon level, talk with Badelaire and brainjack him.

Goulnaba:

Brainjack Goulnaba in level 2

Lee Fei Shan:

Enter the bazooka room in the Hong Kong level. Go up the stairs to the left and brainjack Lee Fei Shan.

Marguerite:

Complete the Vienna level with Rumrod. Use the panel and defeat Marguerite.

Rumrod:

Brainjack Rumrod in the sewer area of the Amsterdam level.

Sam Smith:

Complete the London level with Badelaire, then brainjack Sam in the tower.

Shaja:

Complete the Taj Mahal level with Lee Fei Shan, use her memory to enter Shajas room, then defeat him.

Youthfu:

Defeat Youthfu in the Mecca level.