Hi grase - welcome to the RPF! I don't know anything about the character as I don't play LOL, but looking it up online I found a couple images
Looks like a typical white ninja setup with some cool addons! Luckily, the addons seem like the type of things that will be fairly easy to make in and of themselves!
(I won't explain here, but these bullet points should give you ideas what to search on!)
- For the chestplate, you can use thick EVA foam, like "camping mats"
- For the gauntlets you can go heavy EVA or even Worbla (it's a sheet plastic you can heat and bend/mold, then it hardens when it cools)
- The arm blades and the giant shuriken on his back, you want something stiff but not brittle or sharp (for safety reasons) - you can use a sandwich with a stiff core and more flexible outer material - like heavy cardboard, worbla, or possibly even acrylic/plexi in the middle with craft foam (thin EVA) or something on the top and bottom, extending far enough out past the core that the whole "edge" is just the softer stuff.
- Shoulder ropes can be just regular rope, but it's pretty thick so it might be hard to find/buy, heavy, and tough to work with.
Hope this helps and welcome to the cosplay addiction!