In your example: "I am the one that is here", you correctly placed the emphasis on the identity of the person who is "here", on this predetermined spot. However, your alternative inside brackets ("I am here, but where are the others?") is incorrect on this context, because here the taken-from-context emphasis is on the location of a given person (i.e., the exact opposite of what is intended). On the first case, the location is predetermined and one wants to know who is the person there, on the second case the person in question is fixed and his location is the requested information. The topic, marked by は, refers by definition to the UNrequested information.
In your example: "I am the
In your example: "I am the one that is here", you correctly placed the emphasis on the identity of the person who is "here", on this predetermined spot. However, your alternative inside brackets ("I am here, but where are the others?") is incorrect on this context, because here the taken-from-context emphasis is on the location of a given person (i.e., the exact opposite of what is intended). On the first case, the location is predetermined and one wants to know who is the person there, on the second case the person in question is fixed and his location is the requested information. The topic, marked by は, refers by definition to the UNrequested information.