I've read that The Tale of Genji, and similar Heian-era novels such as The Pillow Book, and The Gossamer Years were predominantly or exclusively hiragana, which is also called "women's writing" (女手). ...
Nowadays, katakana tends to be used for gairaigo and onomatopoeia, while hiragana tends to be used for native Japanese words. This is a slight simplification - more information is available here. ...
When looking at the hiragana ぬ (nu), ね (ne), and る (ru) one notices a small circle in the symbols. In fact that circle is the only difference when comparing them with the hiragana め (me), れ (re) and ろ ...