Just /dl uke

everybody can see?

This no only change name, this too change skin.

You can't change Uke's items for others to see, just for yourself in the custom folder. You could however use Uke's local 'outfit' in a video. The only other way really is to use someone else's tori, ex. Hampa or etc.

how come Oblivion does it?

go to Toribash folder
search for replay folder
open the file using Notepad or sumthing
you'll see some text
change Uke name into someone you like

Oblivion fights Oblivion, not Uke.
To do as he did, just follow Hucota's/tricerafi's instructions.

You can't, just change Uke's textures and expect others to see this change. That way, anyone could edit Uke and that would be terrible.

Just write someone else's name into the file instead of Uke.