Is a Last name on a character needed tomake them believeable

[Miz Lina]

Is your character a person? Because people have last names. Does your character's last name have to be mentioned in the story? No. Just like you don't need to tell the reader the s/he is 5'3", you dont need to tell the reader her/hisi last name. But you do need to know, for your own consistency, an estimate of how tall the character is, because people have heights. Just like people have last names. Stories have been written without the reader ever even knowing a character's FIRST name (Fight Club) but you can bet your house that the character still had a name, even if no one but the writer knows it.

Also: Real people very rarely have relevant or 'clever' last names. Clever last names in fiction is nothing more than a gimmick.

[Lorret]

No, last names aren't important.
I have many characters running around without last names. Sometimes I don't even reveal their first names, just nicknames.
If the last name isn't of any importance to the plot or the characters development, don't bother mentioning it. You can give them one if you like for your own sake, I've done that with a few, but it's not shared in the story because I don't think a last name is important.

[Mishteh]

I actually find that last names can be very important and absolutely necessary depending on the story, though sometimes they aren't, I never leave my characters without last names. Namely because I delve into the character completely. Wondering about their first, middle and last names, how they feel about their names, how it's effected them throughout life and so on.

I have a few methods for last names, the first being most common last names websites where you can get an idea of what you want to last name to sound like without going into too much work. Then if it's too common, variate the last name to something believable and unique.

The second option for me is random words. I was trying to come up with a character's name while I was in school, and I had a really, really hard time deciding. I heard another student call out to another girl and I liked, to some degree, the way her name sounded so I switched the vowels to create something more personal.

As for her last name, on the board it said "Spelling Test Thursday"
I took Thur from Thursday and Ling from Spelling and I had Thurling. It's so easy to say and the spelling is so simple that it passes, perfectly, for a last name in my opinion.

Good luck deciding, or not.

[Alberic of Krufton]

Depends on the setting. Sometimes people are called just by their last names (military, school teachers, British schools I think) so it becomes one of the ways we identify the character. As long as we know who they are, any kind of name (or even no name at all) will suffice. Could be a first name, surname, nickname, simple physical description or other epithet. Doesn't really matter which.

Now, if in the setting a surname is important, (ties to family, formal language and forms of addressing folks, et cetera) then yes, pile on as many names as you need. The name itself isn't as important as what it implies in the narrative itself. This can be culture, social rank, family history, anything.

In short: use names as they're needed. If you insist on giving every incidental character a full set of names when you don't even address them personally in the story, well, that's just extra work.

[I Go Through Worlds]

it depends on how long the work is.

it may be a little weird to read a whole book about a character, and not have their last named mentioned even once.

[Lord Tezzy]

I only give nobility surnames in my current work. Everyone else has one, sure, but it doesn't matter, nobles, though, their names matter. A lot.

If you absolutely feel the need to give everyone last names, then either go to behindthename or learn how they actually developed (father's job, hometown, your job, father's name, etc.)