(I don't consider these spoilers because it's basically lifted from novel descriptions)
Ami is Kitamura's childhood friend. She's a profressional model and is pretty, sweet, friendly etc. Except that the impecculate exterior is an act - she's really quite mailicous deep down. Only Kitamura, Ryuji and Taiga knows about her true personality. She and Taiga instantly became arch enemies on their first meeting when she provoked Taiga (not smart ^^
with her true personality.
She's arguably the more interesting main female characters out of the three as her true intentions are never quite clear. Also not only does she plays an important part in providing a lot of comedy (her verbal arguments with Taiga are insanely crazy), but she also furthers the relationships between the characters. She's a catalyst character in the story so her seiyuu's ability is quite important.