To protect your data you have to have key, of any sort, which you can guard and control physically and be separately from your secret data. Password in your brains is example of such solution - it is physically guarded by owner, by memorizing it.
As password there might be some visual token for your super robot, like piece of paper with barcode which decrypts your software and allows it to run that way. It may worth to consider, as same piece of paper might be used to shutdown your creation, with help of openvc and scripts of course.
Less practical but easier way to implement, is to have 2 sd cards, one contains key and is used to boot the system and waits until you change sdcard and will continue to boot after that.
you should look in to how initramfs is used to boot whole system. You should look how to create custom initramfs, and be able to modify init script for waiting and decrypting mounting your second card with main system.
System is not perfect, but dirty and cheap.
It also do not have to be initramfs solution, you might use solutions like copying your executable codes to ramdisk from some usbdrive, which you insert just for purposes to launch your creation, and remove it when it begins to be alive.
When you have secret and key on one media(is that smart trick, hidden something etc) it can be reversengineered.
As for me initramfs solution needs less effort to make, plenty of information how to make custom initramfs's.