Travelling from Otsuki to Kawaguchiko

I will be travelling on a Sunday in October from Shinjuku station in Tokyo on the Narita Express train (with a JR Pass) to Kawaguchiko station. According to a travel app I am using, I have to stay on the same train at Otsuki station to get to Kawaguchiko station (Fujikyu Line).

Will I need to pay for another ticket for the journey between Otsuki and Kawaguchiko? If yes, where/when should I purchase the ticket as I will be staying on the Narita Express train?

You need to transfer trains at Otsuki, from JR to Fujikyu. Your pass is not valid on the Otsuki to Kawaguchiko train ride. You must purchase a ticket at Otsuki.

I apologize....I did not read the N�fEx portion. You just stay on your train to Kawaguchiko.

Will I need to pay for another ticket for the journey between Otsuki and Kawaguchiko?

It depends what JR pass you are holding. If it is the Tokyo Wide Pass, then the entire journey is covered. However, if it is a different pass, such as the Japan Rail Pass, then you will have to purchase a separate ticket for the Otsuki-Kawaguchiko section. You may be able to do this at Narita Airport at the same time when you get the seat reservation. If not, then you should be able to pay the difference at Kawaguchiko Station or to the conductor on board between Otsuki and Kawaguchiko.