Board Thread:Questions and Answers/@comment-31649453-20170401234321/@comment-27701762-20170402000141

Strictly speaking it is a possibilty, in the sense that insofar as Burgerpants has a real name it could be anything.

But if the question is rephrased as "Is 'Kobi' Burgerpants's real name according to the game and/or its canon?", then the answer is "No." Nothing in the game's dialogue, sprite files, or the Undertale Art Book suggest that there was any real name intended for him, much less that that real name is "Kobi." I can't find anything else suggesting that it was his real name, so I'm guessing the author of the fanfiction simply made it up for one reason or another.