To add to this rather interesting discussion of character design copyright, I have some firsthand experience seeing what goes on behind the scenes with trademarking characters because in my day job, I work for a children's media company who want to build a franchise around their flagship character.
One of the most important things to do if you want your character to be trademarked to use on merch is make sure their design has "unique identifiers". These "unique identifiers" are distinctive visual characteristics of your character's design that are specific enough to point to in a court of law. For example, Steven Universe is a short boy with black curly hair drawn in a specific way and he wears a pink shirt with a yellow star on it, turned-up jeans and pink flip-flops. Both the characteristics and the way they're drawn can be unique identifiers, for example, You can't say that any balding chubby guy in a white polo shirt and jeans is a Homer Simpson ripoff, but characteristics like his two hoops and a zig-zag of hair or his bright yellow skin are definitely "unique identifiers" that could be used in a court case if somebody made a suspiciously similar character and put him on merchandise.
Not every character needs unique identifiers, for example, many slice of life, romance or drama comics feature very generic characters, because their business model isn't really built around merchandising the characters so much as monetising the story people are emotionally invested in. For action and adventure stories though, the "hero" is a big part of the appeal and so as basically being almost the logo for your series, they need to be "legally distinct", especially for children, because here's a fun thing to look for: Shows for young kids, nearly always have the character as the logo (like Peppa Pig) because young kids can't read well and are hyper-focused on the main character, so when they see the character on merch in a shop, that's what they'll recognise and point to and ask their family to buy the item.
If you're selling merch of your character and your series gets popular, there's a chance somebody will make a ripoff, and then your lawyer will need to stand in a courtroom and point on an image of your character and the alleged ripoff, the specific features that match and why it's not plausible they're just sort of similar or generic enough that loads of characters might have that combination of features. If it's too vague or generic a similarity, the case will get thrown out.
For this reason, kids media franchises put tons of thought into the "unique identifiers" of their designs; things like colour combinations, accessories, shapes and even things like how the character's mouth or eyes are drawn. It's a good thing to put some thought into when designing characters if the characters are a big part of a series' appeal.
