[Evil Arms]

I found some information that you may find useful; credits go to Skeetendo for this information:

In Pokémon Yellow, the player receives a special Pikachu that shows emotions, in a primitive form of the Happiness factor in G/S/C.

• Gaining a level
  • +5 if current happiness < 100
  • +3 if current happiness < 200
  • +2 otherwise
• Using, e.g., a Potion
  • +5 if current happiness < 100
  • +3 if current happiness < 200
  • +2 otherwise
• Challenging a Gym Leader
  • +3 if current happiness < 100
  • +2 if current happiness < 200
  • +1 otherwise
• Depositing in PC
  • −3 if current happiness < 200
  • −5 otherwise
• Fainting
  • −1 (always)

There are two events in Pokémon Yellow that are based on its happiness. The first is on Pewter Museum’s top floor—a man whose daughter wants a Pikachu will say different things based on how happy the player’s is.
If happiness ≤ 100:

Your Pikachu looks untamed. May I have it for my daughter?

If happiness > 100:

I’d like to get that Pikachu off you, but it’s too attached to you.

The other instance is in Cerulean City. The character Melanie will reward the player with a Bulbasaur if Pikachu’s happiness is 147 or higher.