Hey You Pikachu/WMG

Flowers are part of a pikachu's natural diet
Unlike other non-food items in the game, flowers don't make Pikachu react negatively when he eats them. Furthermore, when you bring him into the backyard he eats every flower in sight (completely ignoring the player until he's finished). Pikachus instinctively prefer the yellow ones because the pigment helps color their fur.

Hey You Pikachu was a testing ground for the gameplay that eventually became Nintendogs.
Kinda meta, here, but hear me out: in both HYP and ND, we have adorable virtual pets who can be spoken to with voice recognition technology, but they each do it a different way. In HYP, the user's voice is matched to pre-programmed data, which needs to be meticulously programmed for different accents and languages - the reason why HYP took two years to come Stateside and why it never made it to Europe. Nintendogs, on the other hand, simply records the user's voice enough times to "learn" - one reason why it managed to become a runaway international hit.

The technology as Nintendo uses it had to start somewhere. HYP was a test for the pre-programmed version; when it didn't work too well, Nintendo sat on it, then brought the learning version to bear after Miyamoto got a puppy.

Nintendo made the game by crystalizing cute and using this to print the cartridges.
...yep!

Pikachu only understands the truth.

 * Words that Pikachu understands are represented by red text.
 * In Umineko no Naku Koro ni, everything written in red text is true.
 * We can conclude that Pikachu can understand the truth.