Select the eight option at the main menu, "Settings", then select the bottom right section, "Password". Enter one of the following passwords to activate the corresponding cheat function.
Play as Himura Battousai
Enter hitokiri as a password.
Play as Shinsengumi Saito Hajime
Enter miburo as a password.
Extra points
In the Options menu (upon entering the main menu, go up twice), go to the Password screen (the second option in the top row) and type the following password.