Pause the game, displaying the Options Screen.  Then, select the "Button Configuration" option, highlight "Aim", hold R, and press Z (10x).  If entered correctly, the white boxes are "Config" and "Aim" will turn red.

Extreme Battle Mode

Complete both of either character's scenarios with an "A" rank.  Then, find this option under the "Special" menu.

Play as Ada Wong (in Extreme Battle Mode)

Complete Level 1 of Extreme Battle Mode with any character.

Play as Chris Redfield (in Extreme Battle Mode)

Complete Level 2 of Extreme Battle Mode with any character.

Play as Hunk

Complete both of either character's scenarios with an "A" rank.

Play as Tofu

Complete both of either character's scenarios with an "A" rank, and the Hunk's scenario with any rank.