The man Rin calls the "Professor" (or "Sensei" in the Japanese episodes) lives in a tent in the park. While not much is known about him, it seems as though the man lost his entire life savings on bad investments when Japan's post-war economy went sour during the late 1980's-mid 90's, which led him to his current circumstances.
Rin goes to the Professor to ask about the Sutherland's stamp, and he seems to be an expert in the matter. After showing her Japan's first stamp, a prized possession that he wouldn't sell (even though selling it would give him the money he needs to survive for years), he goes on to tell her that no one has ever found a full sized version of the stamp she is looking for. What he does not mention, however, is that he already is in possession of said stamp. It appears as though he is ashamed of this fact since his pride over it prevents him from gaining back everything he had lost.
The Professor later gets attacked by Laura and the stamp is stolen from him, but, by the end, Rin is able to recover and return his prized possessions to him.