Angel - "Buffy the Vampire Slayer" and "Angel"
Here's a loner who's actually more of a victim of circumstance than anything else. Angel lived lifetimes as a murderous vampire before being cursed with a soul and driven to the life of a reclusive private detective to try and atone for his misdeeds. To make matters worse, the curse also contained a stipulation to where if he ever experienced a moment of true happiness, his soul would again vanish, causing him to again wreak havoc on those around him. With a backstory that interesting, we're starting to understand a bit what makes these loner characters so intriguing.