Prof. Elm

In the Games
Prof. Elm's role is the same in HeartGold and SoulSilver as it was in Gold, Silver and Crystal. At the beginning of these games, he asks the player character to run an errand for Mr. Pokemon. He says that he/she may take one of the three starters (Chikorita, Cyndaquil and Totodile) as protection on the way. While at Mr. Pokemon's house, Prof. Oak gives the player a Pokedex so he/she can collect data on the Pokemon of Johto. Once the player returns, he/she is told that someone has stolen one of the Pokemon, the one with the type advantage over yours (if you chose Chikorita, then Cyndaquil will be stolen). The player actually battled the thief before entering the lab.

Chikorita
Chikorita is one of the three Johto starters that can be chosen by the player to accompany him/her to Mr. Pokemon's house. If the player chooses this Pokemon, then the rival will later steal the Cyndaquil.

Cyndaquil
Cyndaquil is one of the three Johto starters that can be chosen by the player to accompany him/her to Mr. Pokemon's house. If the player chooses this Pokemon, then the rival will later steal the Totodile.

Totodile
Totodile is one of the three Johto starters that can be chosen by the player to accompany him/her to Mr. Pokemon's house. If the player chooses this Pokemon, then the rival will later steal the Chikorita.