This movie seems to be edited together from episodes of a serial -- it jumps from sub-plot to sub-plot, and introduces a whole raft of new characters without developing any of them. It's Shin-Toho Studio's entry in the Superheroes from Space genre, which puts it side-by-side with those MST3K staples, "Invasion of the Neptune Men" and "Prince of Space".
Like those other movies, this one is frequently laughable. Jump cuts are used to make it seem as though the hero, Starman, and his monster opponent are jumping great distances... but the effect only works if you're under the age of ten and in a very forgiving mood. The meandering plot is juvenile; and the opening scene features an interplanetary council of some of the most hilariously unlikely aliens you'll ever see. Worst of all, I find Starman an uncharismatic and slightly stuffy hero.
What distinguishes "Evil Brain", though, is its monsters, which are genuinely disturbing: one is a swift, chattering beast, with glaring eyes (one of which is on its stomach), huge fangs and long "cobalt nails". It also has two filigreed crests on its head (like bizarre Mickey Mouse ears), breathes smoky radioactive fumes, and has the ability to turn invisible... The other mutant is a silent woman whose face resembles a beaked bird of prey. She also has special skills: she can turn invisible, and she kills with one touch from her fingertips. In fact, she kills a young mother while her two small daughters are playing (a scene which probably explains why this movie doesn't turn up on TV any more!).
Finally, as I write this, I note that IMDB recommends I might also enjoy "Tanin no Kao/The Face of Another". Hmmm... I'm afraid if I followed "Evil Brain" with Kobo Abe's intense, disturbing, and very mature fantasy, my head would explode.