Insert coins into the HOTD2 machine. Then, click on L two times, R 3 times, L once, R once and press Start before demo begins. (Left for P1 Trigger, Right for P2 Trigger)
Access Training and Boss Modes: Beat the game in Original Mode
Bruno Costume: During original mode in the room where you fight the Emperor, the last room to the left (shoot the door). Caroline Powell from the Dynamite Deka series will give you it.
Display Score: Left, Left, Right, Right, Right, Left, Right, Start (on title screen)
Rogan Costume: Go through original mode, but at the begining select the 2.0 bullets and Double score items. Play through original mode, rescuing all civillians except for the one on level 4 (the guy the Zombie is holding on to). Then beat the game. You’ll end up in a Secret room. In the trunk is a Rogan Costume and an Infinite Credits icon.
Goldman Costume: Beat Original Mode on any difficulty
Infinite Continues: Beat the game in Training Mode and Boss Mode
Red Blood: Beat Original Mode with any difficulty (access after doing so by visiting the Options menu)
Training Mission 10: Beat Training Mission 1 – 9 on a difficulty of 3 stars.
Training Mission 9: Beat Training Mission 1 – 8 on a difficulty of 2 stars.
Two More Sessions in Boss Mode: Beat all bosses in Boss Mode and get a ranking with each, to unlock Emperor and Fight All Bosses Mode
Unlimited Credits (Original Mode): Complete Boss Mode with 5-star marks for each boss, including Fight All Bosses Mode
Original Mode Item Guide
Click the following video to find ways to unlock several items for Original Mode.
Credits to KAZ Gun-Shooting for making the video guide