The NH-18 are essentially a mass of nodal matter capable of swiftly altering its shape. The nearest sci-fi application I can give is the liquid metal T-1000 from the Terminator movies. It works on similar concepts as the hemosynthetic organs nekos can create for themselves, but they weren't blood so much as... I'm actually not sure what they're made of.
Plot-matter! *handwaves!*
You could have it disguise itself in a room as a computer console, and then when a target gets close, it could flow almost wavelike at it, cresting around just as it's grow bladed edges, spikes or what have you before it fell upon its prey. They're kind of nightmarish to brawl against because they aren't only confined to their original matter - if they gain a hold on something, they'll quite readily begin assimilating it and turning what they strike into more of itself.
Considering how I saw scenes where it showed up a lot like JAWS, one of the bigger battles against it had me use more tangible shapes to show how it turned an assimilated M6 Daisy power armor into all sort of terrible stuff until I gave it a toothy maw, crab-pincers with guns, and tentacles... and then called is a
"sharktopus" from there on in (which is why my players sometime see me throw something nasty at them, and then go "well, at least it's not a like the sharktopus, we can do this").
The thing was nigh impervious to most kind of kinetic-based attacks (at least those capable of being delivered by power armor away teams) but could be susceptible to energy attacks, especially stuff coming out of a plasma flamethrower (neko regeneration is greatly hindered by burns, so, kind of a similar limitation).
