Why did Nadeko use different names for Araragi? Episode 12, the story about Otorimonogatari, contains the story Nadeko Medusa. Nadeko said this:

Favorite big brother: Koyomi Oniichan
In Love with: Araragi Koyomi

why she said it differently? Does it mean that she consider Araragi as two different person? Because in Monogatari Series Second Season she turn out to be a

 yandere, and want to kill Araragi, the man that she love.
The difference emphasizes the extent of her love towards Araragi: she loves Araragi as a big brother figure as much as she loves Araragi as a man.


  Favorite big brother: Koyomi Oniichan


This is the love of a sister for her big brother. Sibling love.

The honorific onii-chan shows that she holds Araragi very dear and sees him as if he were her real big brother.


  In Love with: Araragi Koyomi


This is the love of a girl for a boy. Boy-girl love.

The absence of honorific, which is a stark contrast to the previous line, shows that she loves Araragi as a member of opposite sex, not just love between sibling.