On the Battlefield
A rogue is designed around its secret identity ability, which is triggered very easily. Its special ability is to transform into another unit. However given the opportunity to attack before transforming can be powerful in the early ages. The rogue is considerably strong in the game until the Postmodern Era, where units of that age have much higher defense.
The rogue itself has a defense of 1, so almost any attack will result in the Rogue being depleted of its hit points. On depletion of its hit points, one of two things will happen: either it transforms (using its secret identity ability) or it vanishes from the battle (killed.) On transformation it becomes a different unit, and may retaliate towards the attacking unit.
The transformation turns a rogue into another friendly unit on the field at the time of battle. The transformation requires other friendly units other than rogues to be on the battlefield. As a rogue cannot be transformed into itself, a battle with only rogues renders the secret identity useless. They cannot transform into units from the opposing force, either. If the rogue cannot find another unit to transform into it will simply be killed and disappear from the battle. The unit it will transform into is a random unit selected from the friendly units.
A transformed rogue will appear as a different unit on the battlefield. It will take on all characteristics and abilities of the unit it becomes. It's possible to differentiate a transformed rogue from regular units by looking at the unit properties. One will notice the secret identity ability in the properties of a unit which no longer takes on the appearance of a rogue. But the unit with secret identity is a rogue.
Whatever the rogue transforms into during battle, it will not retain its new identity past the battle. In your army management it will still be a rogue. In each and every consecutive attack, it will be a rogue at the beginning of battle. This means a rogue will start in the second wave of a two-wave battle as a rogue in Guild Expeditions.
Damage and hit points
A transformed rogue sustains its damaged after the battle. Each hit point takes a rather lengthy time to heal. If a rogue is sent into battle with reduced hit points, the damage will be carried over to the unit it transforms into.
Rogues and the Rogue Hideout are featured in most of the events. The Rogue Hideout is also possible to obtain as a reward in the Daily Challenges and from the Challenger's Chest after filling its progress bar.