ive been thinking of using that system as part of my rpg system. would using it be plagiarism?
Plagiarism, Copyright, and game design are three very different things.
Plagiarism is using someone else's work as your own. It's not a crime, unless you do so to commit fraud, in many nations. Paraphrasing the wizard of Oz doesn't make it any less plagiarism...
Copyright is a system of protection of text, and in some nations, certain key characters and plots. Completely paraphrase AGOT, and it might not be copyright infringement, but it's still plagiarism.
Game design, at least under the theory in use by the US Copyright & Trademark office, is a case where best practices includes pulling mechanics from others works into novel combinations... which is part of why games' mechanics are unprotectable via copyright in the US.
For that matter, the four forms of "intellectual property" are all different...
Copyright protects the words, but not the ideas. It also can protect look and feel to some degree.
Patent is processes and devices... the how. It's short lived in the US.
Trade Mark is a protection of names, logos, and product identity. It can overlap with character protections under US Copyright - Mickey may not be under Copyright protection anymore, but he's still a trademark of WEDCo...
Trade Secret usually is either some formula or some process; it's protected by several states' laws in the US, but only by contract under Federal law... I don't know jack about how it exists in the rest of the Anglophone world.
I do know that Creators' Rights in France and Germany are quite different from US copyright, trademark, and patent...
The Roll & Keep mechanic was originally published in the US, where it legally cannot be protected by copyright. (blackletter in the law exclusion.)
It could possibly have been protected by Patent, as it's a process, but John Wick didn't do so. And several other game engines use variations on it, some of which (4d6k3 for attributes, at least as far back as AD&D 1e DMG) predate Wick's L5R 1e... for certain purposes. And given the nearly 30 years since, any such patent would have expired already.