Each playable character requires a certain nature that is determined by the questions you ask at the beginning of the game. The natures listed are then divided by gender, so if it is decided you are a Jolly type, then you will be either a Squirtle or Totodile, depending on your answer to the question "Are you a boy or a girl?". You can review the required natures for each Pokemon on Psypokes.
For Pikachu, you need to get Impish if you are playing as a male or Hardy as a female.
To achieve an Impish nature, answer the following questions with the selected answers indicated by the bold text:
Do others often call you childish?
~ Yes
~ No
Are you a cheerful personality?
~ Yes
~ No
Somebody calls you "weird, but funny." How does that make you feel?
~ Happy!
~ Not happy
Do you like pranks?
~ Yes
~ No
Do you like to imagine things for your amusement?
~ Yes
~ No
Have you ever made a pitfall trap?
~ Yes
~ No
Do you hate to be the last person to leave class at the end of the school day?
~ Yes
~ No
Impish means to be inclined to do slightly naughty things, so avoid answering questions that would detract from that.
If you plan to play as a girl, you will need to earn a Hardy nature. Answer the following questions with the indicated answers:
There is a scream from behind a door! How do you react?
~ Yank the door open
~ Scream in unison
You are suddenly locked inside a pitch-black room! What do you do?
~ Kick the door
~ Cry
~ Clean it
Can you focus on something you like?
~ Yes
~ No
When the going gets tough, do you get going?
~ Yes
~ No
Your friend is being bullied! What do you do?
~ Face up to the bully
~ Caution the bully from afar
~ Heckle the bully from behind
Hardy pretty much means you're capable of enduring difficult conditions, so don't answer that would detract from that.
Here's a list of the questions and their answers and how they will benefit your cause. Hope I helped.
Pochu sux.