StitchShoppe said:
What if they turn out to be too similar to an existing character?
There is such a thing as subconscious plagiarism, so they may be getting a little too close to the line without even realising (or intending) it. Anime is strongly conventional, so it's not hard to create similar characters without ever meaning to.
That said, because a character out of context is fairly generic, if you're not alluding to another show (i.e. you're not trying to get people to confuse your shirt with a well known show in order to profit), it should be fine. If the characters that have been created are given their own unique context (a comic or a backstory for example) that could go a long way. They just need to be their own thing.
Usual seek-advice-from-a-lawyer stuff applies, but if they're creating characters in a certain style, but not actually copying ideas from anywhere, it should be okay.
That said, to use a similar example companies like Marvel and DC will sue at the drop of a hat. They've taken people to court over extremely generic things that the general public would all consider public domain. I don't know how far these court cases generally get, but they shouldn't even make it past pre-trial (and almost certainly do).
You don't want to get sued, but you also don't want to miss out on an opportunity if it's fine to do so.