In modern bahasa, raksasa generally means giant or something big or bigger than usual, tend to describe demon or demon-like figure, but not always. Now in this context we surely refer to this giant figure with demonic appearance as in its original meaning.
As far as I know, even raksasa figure always being described with demonic appearance, there are good raksasa and bad raksasa. Example of well known good raksasa figures are the twin Cingkarabala and Upatabala (both known as Dwarapala or Gupala), the door keeper, the protector. In keris hilt context, I believe the raksasa is the protector, the good giant. But we should not forget that even the bad were worshipped in Hinduism so that the bad will be kind and protect the worshipper, which means its possibly the bad raksasa too. I believe it would be difficult to find which exact characters they are in modern explanation, maybe someone in this forum has a better explanation, if possible with older reference.
The face at the back generally regarded as Garuda, the good eagle. I'm not sure about the explanation, but possibly the watcher (of your back).
Now, I will leave the space for someone with a better knowledge within the subject, I would like to learn about this too, it is my favourite subject.
(I add some reference picture of good raksasa from the internet, one showing the garuda head at the back)