I remember when I first started watching Merlin, I was confused about who the characters were. I thought Arthur and Morgana were together (yeah stupid me) But I still think they would have made a cute couple (if she wasn't his half-sister)

Personally, I think their personalities complement each other. Arthur's rather hot-headed and brash whereas Gwen is level-headed and intelligent, so they both level each other out. They both seem to bring out the best in each other too, with Gwen Arthur acts more compassionate and kind and with Arthur Gwen is more bolder. So if that's what you meant by them "suit[ing]" one another, then I believe they do. They're certainly the most developed/complex relationship on the show, regardless.

Hell yes!!!!!!!!!!!!!!

I thought they were setting up a Gwen/Merlin thing in S1, then as the show went on I preferred Merthur.