[Kiwami]

I would go with either of the systems that are not integrated into one's body or slowly alter it. Or are bound to some ill-fated destiny like the Ryuki stuff.

I think I would use either Zecters, Gaia Memory or Lockseeds. Since the latter two have a higher chance of being dangerous not only for you but people around you as well I think Zecters are best. Of course, Zecters are also linked to some bad shenanigans, and they can leave you if you're not "worthy" enough anymore... meh.

So I guess Gaia Memory then? But I'd have to make sure that the belt is working properly and lock it away safely.

If I could choose one it's either Skull or Eternal for me.