Character + Harlequin cover movement query

Quick question guys and girls,

Harlequins 'are not slowed by difficult terrain' (Codex Eldar p66) due to their Flip Belts. However this isn't a unique special rule in the rulebook (eg. it isn't Move Through Cover).

Therefore Archons (or Succubi/Haemins/Special characters) that join a Harlequin unit will not gain any of the same ability.

So a unit of Harlies joined by one of our characters WILL get slowed down when walking in difficult terrain  (often the same terrain that gives cover). Right?

No, the unit won't be slowed by the IC. In your example, the Harlies are not slowed but the Character attached is. As long as you maintain coherency you're fine.

The Harlies can move their normal 6''. However, the Archon or IC would have to roll and move the specified distance. As long as you remain in coherency, you are fine.

Think about Jetbike Assault move with an IC attached that is not on a Jetbike. Same concept.

Great, thanks guys - yours answers are helping me to decide which is better:
- Craftworld Harlies (2D6 sight to shoot them, but has to use a warp charge)
- Dark Eldar Harlies (Shrouded + Stealthed)

The Dark Eldar harlies require cover much more than the Craftworlders and therefore will end up slogging through difficult terrain. Thus the Craftworld Harlies seem to be the better option for an accompanying character whilst the Dark Eldar ones make a better solo unit.