Outlining in steps what Maho76 said...
1. Place a trigger zone where you require it.
2. Place the character where you want it.
3. Right click on the triggerzone. Set the main script to "plrinzoneactivateused.fpi". In the If Used field, enter the name of the character entity. Click Apply changes.
4. Right click on the character. In the Name field, enter the name you used in the trigger's If Used field. If you put "Joe" in the If Used field for the trigger, then you enter "Joe" in this spot.
5. Set the Spawn at Start to No. Click Apply changes.
When the player hits the trigger, the trigger will spawn the character.
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