This could have been a subset discussion of the Crowe-Rapier question. The chapter on Characters does not seem to very specifically detail what does or does not count as a character. Or rather it explicitly lists sergeants and IC's as 'characters'. However it goes on to list Mephistion who is clearly listed in the blood-angels FAQ as a 'single model entity' and neither a sarge nor an IC.

I'm of course most interested in how this relates specifically to Tyranids. The Tyranid Prime, and the PoM are pretty obviously IC's... but what about the Swarmlord? Do any of the 'unique' models count as Characters? What about un-named hive tyrants joined to a unit of tyrant guard?

Perhaps I missed this being enumerated elsewhere.