From Bulbapedia, the community-driven Pokémon encyclopedia.
Mr. Pokémon (Japanese: ポケモンじいさん Old Man Pokémon) is a non-player character who lives in the Johto region. He casually researches Pokémon findings as a hobby, and is old friends with Professor Oak. He keeps in contact with Professor Elm because he knows that Elm is the authority on Pokémon evolution.
Relation to the plot
At the beginning of Pokémon Gold, Silver, Crystal, HeartGold, and SoulSilver, Mr. Pokémon contacts Professor Elm about a Pokémon Egg he has found. In Generation II, this Egg is notably one of the first Eggs to be found, but in Generation IV (to tie in with breeding in the games set before HeartGold and SoulSilver) it is just "another Pokémon Egg". Elm sends Ethan/Kris/Lyra to collect the Egg on his behalf, and later entrusts the player with the Egg. It eventually hatches into Togepi and is revealed to have been given to Mr. Pokémon by the Kimono Girls.
Later in the games, in exchange for a Red Scale, obtained from either catching or defeating the Red Gyarados in the Lake of Rage, Mr. Pokémon will give the player an Exp. Share.
In HeartGold and SoulSilver, after obtaining both the National Pokédex and the Kanto starter Pokémon from Professor Oak, Mr. Pokémon will give the player a Blue Orb in HeartGold or a Red Orb in SoulSilver. This will activate the Embedded Tower event.
In the manga
In the Pokémon Gold and Silver: The Golden Boys manga
Mr. Pokémon appears in the Golden Boys manga where he gives Gold an egg as an errand for him to deal with.
According to Professor Elm, "Mr. Pokémon" is merely a nickname; the character's true name is unknown. As a researcher, Mr. Pokémon takes pride in the discoveries he makes, although Elm suggests that these are not as exciting as he would make out.
His house, located at the northern end of Route 30 suggests that he travels a great deal since he has a large collection of foreign magazines and coins. After the Embedded Tower event, Professor Oak cites Mr. Pokémon's research on Groudon and Kyogre, indicating he may have traveled to Hoenn in his past.