So I've recently found Luigi in Super Mario 64. Here's how you unlock the very-well hidden Luigi:

1. Start a new game on file B.

2. Get the first 2 stars in bob-omb battlefield in less than 5 minutes.

3. Go to the courtyard.

4. Run around the statue 64 times.

5. After the 64th lap, swim towards the statue (from the front) and punch the blurry text.

6. Luigi should appear if you did this correctly.

7. Quickly talk to Luigi.

8. Luigi will decide to join Mario on his adventure and reveal 5 more secret star levels.

Note: You can switch between Mario & Luigi at any time, so you don't have to go back to the courtyard.

Hope this Luigi guide helped you!