The character's first name is stored in that character's character IFF ("user000xx.iff"). To find which character IFF belongs to which Sim, use a utility like Character Viewer, or open all the character IFFs in IFF Pencil and look at the name in the CTSS or OBJD blocks.
Last section showed how to copy a character's bio to all languages in the CTSS section with IFF Pencil. The name can be changed in that same screen; it is simply string #0 in the CTSS. (Don't forget to set for all languages.)
Or, if you're working on a Mac, there's IFFsnooper.
The name is also recorded in the OBJD block. The name in the CTSS block is the one that is displayed and needs to be changed.
Other utilities can be used to change the name too, provided they work with the installed version of the Sims. Like this one:
Courtesy of Sims Bodyshop.
If I really want to (because I'm playing a version of the Sims so new that no utility can work with it?) I can change the name in a hex editor; UltraEdit, for instance. This is a two-step procedure and only really works if the new name is shorter than the old one. To change SailorV to Minako, for instance, I first locate the IFF file.
Found you!.
Then I open the IFF in the hex editor, search for the old name (note that I checked "Find ASCII")...
UltraEdit in action..
and type the new name over the old one. If the new name is shorter, I change the remaining bytes to hexadecimal 00.
Yes, right there.
The new name should not be longer; adding bytes will mess up the internal structure of the IFF file, and overwriting 00-bytes that separates sections may cause them to run into each other.
Oops! The bio is now part of the name.
When editing hex, I simply change the name wherever it occurs (in at least two places: the OBJD block and a CTSS block for every language used). With one file, though, this has given me a bad user IFF which crashed the game every time I loaded the neighbourhood. So I had to change it in two steps: first the OBJD block name, then load and save the family, then the CTSS block name. The house will complain of a "missing object" - this is in fact the changed name. Maybe the safest option is to leave name-changing to a proper object editor...
Like the incredibly useful IFF Pencil.