I prefer introducing characters progressively with time. I think every character should have their entrance and proper introduction, basically their time to shine 
It's true that some readers might not grow fond for a new character immediately.
But I rather risk that than shoehorn in my cast and overload the story or have an unnatural introduction.
Does the plot call for them to be introduced early? Then introduce them early. Later? Then introduce them later. When you go to write a story you must write it with the intention of being an author not a fan or else what you end up with is a pile of fanservice.
Characters can be introduced at any time during the story, as long as it's written well and makes sense. For this, I advise reading some stories or re-reading previous stories that you enjoyed and breaking down what those writers did. For example, in the series "Red Queen," the writer introduces new characters even in the third book of the series, and so does the writer of Game of Thrones.
As the writer, you decide when new characters should be introduced into the story. You also have to consider setting and place. If your characters go to a new world in book four, they'll obviously be meeting new people. Of course, the main character should be introduced at the beginning of the story. I hope that helps!
I think it depends on the story. With (Semi) Super2 , i introduced my main characters early, but my reoccurring characters will be introduced later.
I like to see the mains early on just because I follow stories mainly because I like character interaction. If the characters are strong, I'm in.
Depends. If the story is character driven, introductions are going to follow a character set pacing rather than one that is to serve a plot driven story
It might help to break down the story into arcs to determine when a character should be introduced. If a new character is critical to a relationship with an already established character, having them enter when an arc focuses in on that established character can make it cleaner
The same approach can be useful for plotting arcs. If a character is a main antagonist, introduce them in keeping with setting conflict for the main arc. If a character is a secondary antagonist, introduction can wait until the main arc starts to thread out into smaller pieces and then be woven back in as needed. The character introduction serves to direct the plot
And reader attachment is not necessarily correlated to time spent on screen. Some characters are people you travel with for the whole story. Others are fireworks, there in a flash and gone with a bang but leave the reader wishing for more
I think if you're aiming for a more traditional webcomic style, introducing the characters early on is the way to go, since it gives readers a sense of whether they want to read it or not.
But if you want to go for a more graphic novel or comic style, then it's best to introduce the characters naturally as the story goes on. It's a bit more realistic too in my opinion.
I know a lot of webcomics do that thing where they have one page with "meet the characters" and it lists their basic personality and traits. I personally would suggest not doing that since it doesn't really allow for character development -in other words, "labeling" your characters from the start may make it harder to develop them later on.
Hmm sometimes I mention them within dialogue but they never make an appearance until later on or just as the story goes on 
I introduced a good chunk of my main characters pretty early on in my comic, but I gradually brought in other characters as time went on as otherwise I felt dumping the entire cast all in one fell swoop would be a bit overwhelming.
Basically get people to like the characters you introduce and then introduce other characters later on, which I always did in story arcs
depends, main cast? should be introduce from the start, villans? can be introduced progressively in the story, secondary characters? after establishing main characters, and to get people invested has nothing to do with how long you see a character, it how it acts and if the audience clikes with the character example kamina from gurren lagann is established early and quickly and we get attached from ep one just by his energy and personality, while trunks from DBZ is established farly at the end of the series and we also clike with him, concentrate on making the characters appealing and when the plot or story asks for them introduce them
The only thing I'd recommend AGAINST is introducing new characters after the midpoint of any arc (whether it be the main arc or subplots). As with objects, introducing new characters after the midpoint can come off as "solution out of a hat"-ish (i.e. deus ex machina). It can make it seem like the character was created solely to get the other characters out of a bind. As in all things writings, exceptions can be made, but it's harder to pull off well.
I don't think that a character that's introduced late isn't attachable. It all depends on how you write that character
One thing to keep in mind of is introducing too many new characters at the beginning could leave your audiences confused.
I think you can introduce them at any time during the story. Just be sure to make it believable, and give the reader a reason to like them. Make them do something nice or good to which the reader will say, "Hey, that's a nice guy" or make them feel for them by putting them in a position to where the reader can relate and feels bad for the character.
I wouldn't introduce all characters at once. It's infodump, and it's better to avoid that.
There's nothing wrong with introducing an important character later, readers can still get attached to them. All you need to do is make them memorable (which is easier said than done, I know). Good luck with your story!
I think it depends on the story. My sis and I have a type of "road trip" comic where our main characters travel around a planet looking for their missing family. They meet a lot of characters on the way who all have their own stories. They'll be introduced when the comic gets to them. But that's just how the story goes. It probably depends on what works best for the story as to when to introduce new characters.
I hate when a lot of characters are introduced at once. Like... You can show 20 characters at once but don't expect me to learn their names at the same time? And don't use them all at once. It's a subjective thing sometimes cuz well I just won't remember randos who mean nothing to me. I like getting to know characters along with the protagonist, kinda. I guess I prefer a focus on smaller casts, with more characters added/explored gradually. By the end, I can know and care for all 20 characters and more. Butyea. Readers will get attached to whoever they like provided they're interesting enough.
^^^ I think this is a good point. In our comic, there are some major players that are introduced past the half way point, but our characters did talk about them earlier in the plot, and their appearance makes sense later in the story as the characters are going on a journey to find other characters. But in general, I think its weird to be forever introducing major characters especially as the series is heading towards the finish. It makes them feel less like major characters and more like convenient background.
I think they should be introduced progressively or when the start to become relevant. I find it annoying when a bunch of characters get introduced in the beginning, only for some of them to be put to the side until the plot needs them and by that time I forgotten about them.
To me it really just depends on when it makes sense for the character to be introduced. Only two of the initial four main characters are shown in the first chapter of our comic for instance, but two more will be in chapter 2. Then there are certain characters that appear to have little importance to the story at first, but as the story goes on they will be integral to the plot.
I actually introduced all four main characters, although two of them briefly, in the first chapter of my current comic. In the second chapter, the briefly introduced character got the limelight and got properly introduced. With each chapter, I usually present a few characters that are secondary to the story, but to help understand the main character's back stories and personalities and such, and I guess that will happen for a few more chapters =)
I've introduced main characters to stories pretty late in older comics, but I think I prefer to introduce them early and flesh them out as the story goes - I've made this character, let me tell you (almost) everything about them XD
