We really need to see some of your work to be able to give advice on that. All these what ifs and vague statements of your characters' sueness don't really give us anything to work on.
And as always, powers and abilities don't make a character a Mary Sue, it's when the plot and the other characters are bent to show how perfect that character is, etc. etc. etc.
Crab King
http://www.mushroomgo.thecomicseries.com
Like I said, the time traveler in question hasn't been introduced yet. While I don't think the plot's been -bending- per se to show off any particular character as perfect, most of the cast has really unpronounced flaws—quirks, yes, but I don't know if their issues are hindering enough. Almost the entire cast is absolutely great at a number of different things, with very subtle, sometimes barely existent flaws to counterweight those. I don't know if the characters themselves are interesting enough to make up for that, or if maybe I'm just worrying too much.
Be warned that the comic does start with "amnesia," something I haven't gotten around to dealing with on-panel yet. Really, I've abused alot of cliches so far, but most of them I don't feel bad about.
The questions for you to answer are really "Do you think of any of these characters as a virtual you?" and "Do any characters treat any particular character with more respect/compassion/admiration than the rest of the characters without reason?"
In other words, sues come about when you insert a virtual you into your work and then tweak the work to flatter yourself.
Also, you've my compliments on your drawing. You've managed to get down the facial expressions right which is difficult.
So, here's my dilemma. Character A is a consistent member of my webcomic's cast. He is established early on, and is one of the more popular characters among my readers.
Character B—to be introduced later—can do everything that A can, but can actually do some of it better. B will be introduced on the villain side, but slowly work his way toward anti-hero status as he reveals a grander agenda. Taken at face value, Character B might be a Mary Sue—they're absolutely superior to a pre-established cast member in almost every way, shape and form, maybe only spared by the fact that they are, in fact, a villain and would HAVE to be better in SOME way to create conflict. Still, that's debatable.
Here's the thing: what if character B is actually character A from the future? Does this make character B's absolute superiority to A excusable? What is A is already bordering on Mary Sue status? What if my entire cast is? It initially hadn't bothered me, but after bouncing the idea off my roommate, he pointed out that character B sounds awfully Sue-ish—and now I'm scared half to death that my entire comic is one giant Suedom. Nobody's mentioned it hence far, though, so for now—for this particular moment—I'm just worried about B.
Hmm, I seem to have rambled.
I planned on removing B from the plot once he'd served his purpose, so as not to have him running around at the same time as A post-reveal... but now I'm afraid it reeks of him being "too good for this earth" and all that. I keep getting into this infinite loop of worrying myself about it.
I don't even know if I make sense anymore.