You can only put Evil Characters on a site if a card's special ability tells you to. Several different enhancements allow you to do this, as well as one of the Lost Souls from the Rock of Ages tins. Usually what happens is the Evil Character can join whichever Evil Character that you choose to block. I'll get an example from the REG:
Lying in Wait
Type: Evil Enh. • Brigade: Pale Green • Ability: 2 / 2 • Class: None • Special Ability: Take an Evil Character from your hand and place it face down on an occupied site. A rescue attempt at that site allows the character to band into battle. • Play As: Take an Evil Character from your hand and place it face down on an occupied site. If rescuing Hero(s) have access to that site, the Evil Character may be banded into battle. • Identifiers: None • Verse: Jeremiah 5:26 • Availability: Warriors booster packs (Uncommon)
