A single favorite webcomic character? God, there's so many to choose from. The internet is way too vast to narrow it down to one character. Dr McNinja, Order of the Stick, 8-Bit Theatre, and so on and so forth. So, I'll settle for a personal top three.
3) Elan from Order of the Stick. His gullibility and goodness of heart are cute, and he is awesome when it counts.
2) Min-Max from Goblins: Life Through Their Eyes [http://www.goblinscomic.com/]. He starts out as your very typical fighter in a D&D setting, a self-absorbed, fight-happy fellow obsessed with fine wenches and experience points. He's also the comic relief of his adventuring group, and this pea-brained mentality extends to his character build; his name is a dead giveaway. The reason I like him so much is because his growth in the comic perfectly encapsulates the development of many real D&D players, the graduation from hack-and-slash obsessed bean-counting power player to an actual, if somewhat reluctant, fully fledged roleplayer.
1) Jim from Darths & Droids [http://darthsanddroids.net/]. I like Dungeons & Dragons. I like Star Wars, and I like webcomics, and this weird amalgamation of all three gives us Jim, the player of 'Qui-Gon Jinn, the cheddar monk' (and later Padmé Amidala, etc.), a geophysics student who plays D&D to shut off his brain and have fun and the series' main source of absolutely insane ideas and other things that make no sense. Still ongoing and still very awesome. His awesome is matched only by Sally, when the DM gives her a carte blanche to describe things; a must-read.