The Mitsubishi Outlander is now in its third generation. It comes with keyless entry and factory immobiliser as a standard but lacks an alarm.
My recommendation is the Cobra AK4615 CAN-BUS upgrade alarm which works flawlessly with the Outlander.
It works with all models including the Outlander PHEV (Plug-in Hybrid Electric Vehicle).

Mitsubishi Outlander GEN 3 2013 >
Here are some photos of what the alarm looks like installed:

Cobra Ultrasonic Sensors in the Outlander
Here is how they look when back in the car:

Cobra LED over-ride switch in the Outlander
Gen 2 Outlander 2006-2012
If you have the Older GEN 2 Outlander then the AK4615 is also a great option as shown below:

Outlander GEN 2 Ultrasonic Sensors

Cobra LED in GEN 2 Outlander
GEN 1 Outlander (Airtrek 2001-2008)
The Airtrek can also use the Cobra AK4615 in PLIP mode or a remote alarm.

The alarm works using the factory remote controls or Smart Key