His place as Arthur's servant means Merlin has to hold back some opinions and always try to be as polite as he can be despite the hell Arthur puts him through on a daily basis. But not when he is Emrys, a scrawny old man who gets a kick out of turning the tables and putting the once and future king of Camelot in some weird situations.
Clark Kent/Kal-El (Smallville)
Clark eventually managed to reconcile his straight-laced reporter persona with the all-powerful hero that will one day be known as Superman but it was a long, winding road -- ten seasons long to be exact. But before we got to that point, we had to watch Clark switch intermittently between his two personalities (Sometimes with the help of Jor-El). And while Clark was only beginning to understand the true extent of his powers while Kal-El had a handle on every aspect of those abilities.
Caitlyn Snow/Killer Frost (The Flash)
We all love Caitlyn but we can't help but be drawn to Killer Frost's charisma. Even the star labs team completely trust her now even though she was in bed with Zoom a couple of season's ago. But can anyone blame them? her fashion sense is off the charts, she rivals Cisco in delivering the best jabs at villains and it looks like she is going to be their most viable weapon against Cicada.
Mary/Typhoid Mary (Iron Fist)
Typhoid Mary is the only other personality the mild-mannered Mary knows about. They started off fighting for dominance, with each one of them doing what they can to keep the other suppressed for as long as they could. Slowly, they both realized that there may be a supremely violent and possibly superpowered third personality that may not be as controlled or rational as the two of them. While Typhoid Mary is quite violent herself, she's at least rational and fights only to keep them both safe.
Angel/Angelus and Willow/Dark Willow (Buffy The Vampire Slayer/Angel)
With his soul intact, Angel is still a competent fighter, but due to a curse placed on him, he loses his soul every now and then, and the demon Angelus takes over the empty vessel. As a result of Angelus' actions through the centuries, he became known as the most vicious vampire in all of history. He's not only stronger, but his intellect is also superior to Angel's, and he has got better one-liners. Willow, while not a terrible witch by any standard, is on a whole other level when she goes dark.
Edward Nygma/The Riddler (Gotham)
From lab geek with an eidetic memory working with the Gotham PD to criminal mastermind. Ed's path to becoming the riddler was a long one littered with the dead bodies of both enemies and lovers alike. This was aided in part by the emergence of his Mr. Hyde personality known as The Riddler.