An ISBN acts as a digital fingerprint for all individual editions of a book. This means that the ISBN for the hardcover edition should be different than the ISBN for the paperback, the ISBN for the mass market, the ISBN for the audio book, the ISBN for the eBook and so on. According to BISG (Book Industry Study Group), every single new edition of a book should have a unique ISBN.
You will still be able to publish your book on Kobo without an ISBN and sell in over 190 countries worldwide as we will issue our own identifier number when it goes on our site. However, this may not be accepted for distribution through our all partner sites (Chapters, WHSmiths, etc.) Having a valid ISBN is the industry standard and lends your works with a sense of credibility. We would therefore recommend it for all of our publishers, but it is not necessary.