Words and names are not in and of themselves Copyrighted or trademarked. You could have a character who is named "Clark Kent" so long as he doesn't bear a distinct resemblance to that one mild mannered reporter who I never see around when Superman saves the day. In fact, this actually plagues Marvel and DC at times as they both publish stories about a character named "Captain Marvel". Both characters are quite distinct, as DC's Captain Marvel is a magically empowered prepubscent boy in the body of an adult sized superhero, while Marvel's Captain Marvel is one of several characters who bore the title, all of whom are cosmically empowered superheroes with some kind of tie to a fictional race of human like aliens known as the Kree. Since they are in no way indistinguishable characters from one another, both companies hold the copyright on the respective character(s) with that name.
However, this isn't the same as Trademark. While both companies hold the copyright to superheroes named "Captain Marvel" only one company holds the Trademark to comic books published with the name "Captain Marvel" on the cover: Marvel (by irony, DC's character predates Marvel's by two decades, however, due to a rather lengthy legal battle where DC (then known as Timely) was suing the original publisher of Captain Marvel, Fawcett, over copyright infringement related to art and ideas in several Captian Marvel comics, when DC gained the rights to their Captain Marvel, they effectively stopped publishing stories about Captain Marvel to bury him. By the Time of Marvel's character's creation, the trademark for a comic book about a superhero named Captain Marvel had lapsed and they saw a prime chance to have a hero named after the company).
To get around the above issue, DC does one of two things: A.) Make their Marvel a guest star in other characters comic books or B.) Release Captain Marvel's own book under the Title Shazam (the magic word that allows the character to become a hero or return to being a child.). And while only can make comic books titled Shazam, the magic word itself is not protected and appears in Marvel all the time (most famously in the first Spider-man film where Peter Parker says it while trying to learn how to shoot webs. In the comics, characters sleep talking tend to mutter shazam alot.).
And in your specific case, both DC and Marvel have characters named Neutron. DC's is probably more famous, being a anti-villain of sorts with powers similar to Capton Atom and a more unhinged personality. Marvel's is an Alien with Super Superstrength and skin that looks like star filled field of space that is part of the Royal Guard of the Emperor/Emperess of the Shi'ar Empire, an alien interstellar empire that most often appears in X-men.