During the mission, capture/liberate Fratelli and enter the stronghold to learn about “a guy with wings”. After entering Fratelli, capture/liberate Coppermine to meet Sheen. Sheen will always ask if there is a place where he can meet ladies, but your answer to this question does not affect anything.
If your Chaos Frame is 60 or lower, Sheen will ask a second question offering to join your battalion. Choose option number 1 (“Well… Okay.” or “…Ummm, sure.”) to add Sheen the Hawkman to your battalion.
This map was ripped by Tropicon and originally uploaded to VGMaps.com. Used with permission. Most neutral encounter information was originally collected by Red Maw. Referenced with permission. Most enemy unit, shop, and stronghold information was originally collected by CyricZ. Referenced with permission.