RPCs are needed to transmit any message from the client to the server, or from the server to the client (one or all).
Including to ask the server to check some data, and to receive a response from the server - the result of the check (callback).
It is the server that must store information about which characters are available to the client.
But this is not their only application, but only one of the options.
You need to build network interaction in such a way that all important data is on the server. And then you distribute this data to clients when necessary.