"How dare you come here and threaten me! Don't think I'm afraid of you, but I see no reason to kill you now. Perhaps you have something of value to say. State your business quickly!"
Orion is a fire elementalist, wielding fire related spells in combat. He is by far the easiest to kill in combat, due to his weak armor and negligible ability to heal himself. The fact that he often quickly gets surrounded and attacked by multiple enemies only makes keeping him alive harder, though said enemies will often take a solid pounding before taking him down. In short, Orion is purely about offense and needs to be defended vigorously to retain his usefulness.
One of the best damage dealing henchmen. Very useful in the Shiverpeaks and Maguuma Jungle, where several monsters are weak to Fire damage.
Lethal when partnered with another Fire Elementalist, especially if they carry Mark of Rodgort. Together, there should be enough AoE to keep enemies burning at all times.
Cons
Poor AI casting. Orion's area spells can be very helpful, but they often go off only after the enemies are defeated. Micromanagement on targeting is necessary to time his spells.
Gets targeted by most enemies and dies easily (weak armor and self-healing).
Less effective after the AI update which causes mobs to run from AoE. Not only will his nukes not do their full damage, but they will cause enemies to break aggro after it was properly lined up. If you will be in tight aggro situations where your party does not have a lot of room to maneuver, leave Orion behind.
Fire Attunement becomes a liability in areas with enchantment removal, especially when the removal would cause damage (Shatter Enchantment for example).
Tips
Try to keep Orion in the back of the fray. Have warriors charge in first and get the enemies' attention. This is because Orion is a favored target of foes. Most monsters will target Alesia first, but those who don't target Alesia will usually go for Orion.
Target a lot of enemies that are bunched together. This way when his fire storm actually goes off, it will affect something living rather than the corpse of an already-slain enemy.
Utilizing snaring skills (such as Deep Freeze) makes Orion much more effective.
"Maybe you've heard of me. I'm famous in these parts for my dazzling smile and my penchant for spectacular displays of fire magic. Women adore me. Other Elementalists want to be me. I'll tell you, it's tough sometimes being so well loved. Now, don't be shy. Just because I'm famous doesn't mean I'm not willing to help a fellow fortune seeker."
According to the game in Spanish, Orion is the same character as Orion Elek; this is further backed up by one of his quotes when you face him in The Norn Fighting Tournament in Eye of the North.
During several of the beta weekend events before Guild Wars was officially released, all of the henchmen had last names, and Orion's was Elek. The last names were dropped when descriptions were added in brackets after the names, to prevent them from getting too long.