Nikolai Lockertsen, who signs his work “Nikko”, is an art director an visual development artist for the film industry, working with Storm Studios in Norway.
His approach to digital painting is often rough-textured and gritty, in keeping with the subject matter at hand, but can also be lighter and more cartoony. He frequently casts his compositions in almost monochromatic color schemes, sometimes punctuated with high chroma spots of the complementary color for dramatic effect.
His website has examples of both his professional work in environments, character development and matte painting, as well as personal work and sketches.
Lockertsen has a number of tutorial videos available on digital painting, and in particular Procreate for the iPad. They are available through Art Study Online, and you can see short trailers for them on YouTube. There are also a couple of longer time-lapse step-throughs (and here).