Congrats on your new reader! In future you're probably going to get better results posting device specific questions in the appropriate sub-forum (Kobo in this case).
In answer to your question I think you might be out of luck. The "Tilde" (~) (not sure on spelling of that) is available on the 2nd page of the number keyboard (press the -+= in the bottom left above ABC) but I don't see the grave accent on it's own anywhere.
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