iI there a way to get the unlockable item after starting a new game?

If you saved the game before with the item yea you will start with it but if you didn't save you have to start over to get it.

Start a new game after u saved it at the end and the RMB should be in the corner just to the left when coming out of the security camera room.

If you saved and your "lil sis" did NOT save over your game then you can still unlock the item.

Items come with the achievements but you >>MUST<< survive and get an ending to unlock the items. If you get the achievement and then die you won't get the item.

Why won't doing another playthough work? Cause you didn't unlock the item on THAT playthrough.

Well then I, Raccoon_Mario22 am screwed for the Special Forces Boots because I got the zombified ending after I got the achievement and the boots weren't there and then I saved over my game in overtime mode because I thought I was underlevelled at Lv. 27 Frank.

Actually it can work, because I just retried Overtime mode at Level 44 and killed ten special forces soldiers then got the ending it gave me the boots.