It is in the nominative.

Just to add to christian and wataya's posts, this is why it is "roter":
http://en.wikipedia.org/wiki/German_declension#Strong_inflection.5B6.5D.5B7.5D

Also, I suggest bookmarking the page. A lot of my questions can be answered using it in combination with the "word formation" option here: http://www.canoo.net/index_en.html