The following question was asked in a French magazine (AnimeLand, issue 71, May 2001) and the English translation (and typed version of the article) kindly provided by Olivier.
AnimeLand: How have you find the idea of Yogurt?
Tamaki: (laughter) Ah, that one! Originally, it is a thing who has been used by the developers for a graphic test. But it became the first character. In fact, I had only doodled it when a programmer said to me: "Give me a drawing, please!". I said to him that I had not even drawn the heroes and that even this drawing was not finished. He told me that it was not important, that anything would work considering it was only to enter graphic data. This is why I gave him the drawing. The name of the character also comes from this phase beta-test: we come across the syllable "yo", by chance and there were only food names, then we came across "yogurt", the name which remained.