The kanji for one thousand (千, sen), appears similar to チ, and at one time they were related, but today チ is used as phonetic, while the kanji carries an entirely unrelated meaning.
Many onomatopoeic words beginning with ち pertain to things that are small or quick.[1]
The dakuten forms ぢ, ヂ, are uncommon. They are primarily used for indicating a voiced consonant in the middle of a compound word (see rendaku), and they don't usually begin a word. The dakuten form of the shi character is sometimes used when transliterating "di", as opposed to チ's dakuten form; for example, Aladdin is written as アラジン Arajin, and radio is written as ラジオ. It is, however, more common to use ディ instead, such as ディオン to translate the name Dion.
In the Ainu language, チ by itself is pronounced [t͡s], and can be combined with the katakana ヤ, ユ, エ, and ヨ to write the other [t͡s] sounds. The combination チェ (pronounced [t͡se]), is interchangeable with セ゚.
Look up ち, ぢ, チ, or ヂ in Wiktionary, the free dictionary.
^Hiroko Fukuda, Jazz Up Your Japanese with Onomatopoeia: For All Levels, trans. Tom Gally. New York: Kodansha International (2003): 19 - 20, Introduction, Words Beginning with ち Chi, Indicating Smallness or Quickness.
