I am a beginner to Japanese and playing Pokémon has got me interested. I am learning Katakana and wondering why in Pikachu it is «ピカチュウ» instead of «ピカチュ». Do you have any explanations or links to offer?
チュウ is similar to チュ but pronounced with a long vowel. Some English speakers seem to have difficulty distinguishing them, but they are very different at least to the ears of native Japanese speakers.
Pikachu, the famous Pokémon, derives its name from a combination of words. As you've noted, in English, we write his name with one U at the end. However, in Katakana, you would write ピカチュウ (pikachū) instead. There are two main reasons why this is.
The first and more important reason, which explains the Katakana, is that ピカチュウ is a made-up word of the combination of the onomatopoeic words ピカ and チュー.
ピカ = The sound a spark makes
チュー = The squeak of a mouse
ピカ coincidentally also happens to be the name of a mouse-like animal in real life and actually served as the original inspiration for the design of the Pokémon ピカチュウ. Functionally speaking, チュウ and チュー are pronounced the same, and it's easy to see the correlation.
The second reason explains why in English it is Pikachu with only one U. Spoken Japanese has a generally short vowel duration, and it is common to see words that have foreign origins such as English have their vowels extended because the sound of vowels in English is thought to be longer than the sound of vowels in Japanese, that is, if it is a two-vowel length sound in Japanese, it can easily be read as a single-vowel sound in English, and vice-versa. Therefore, チュウ and "chu" (in English) have about the same pronunciation length.
ピカチュウ is the long vowel of ピカチュ. In the beginning, you might not see the difference, but with continuous practice, you'll see that the difference in pronunciation is clear. Also, it can be pronounced ピカチュ for the sake of speed talk, but most likely it's going to be pronounced ピカチュー. (It can be written with － or ッ(small tsu).)
Edit: Here's a video explaining it in more detail: Katakana long vowels (YouTube)