Whoa that's alot of questions lol, let's get to it.
First of all, it all depends on how much plot relevance they have to the story. If they're just some "1 chapter" minor OC who just makes the Protagonists help him/her out on something, then you should't worry about what their introduction is like because readers are probably going to forget about them as the story proceeds.
Now if you're talking about major characters that's where it begins to get a bit tricky. Usually when you introduce a new main character, an event happens that has something to do with the Protagonist. Whether it could be Team something attacks the grocery store and that new person help out, or if some trainer is abusing their pokemon and that new person reprehends them. The possibilities are endless so make it something that'll make you go, "Just who is this guy?"
As for their personality and flaws, that's where your social ability comes into play. If your an open person, have a **** load of friends, and go outside occasionally, then try to base their personality on someone you've met in real life. The thing your trying to aim for is making your readers "feel" your characters and synchronize with them. To do this, you're going to need to observe your friends more closely and see how they react, how they laugh, how they talk, and especially how they live. Using common cliche personalities can instantly turn off some readers, all because the author was too lazy to develop a personality above Ash Ketchum.
I would say you need to learn more about the three dimensions of a character and what they are.
For flaws, giving them "forced" flaws is a bad way of doing it because then it just shows that you made those flaws for the sole fact that you didn't want people to suspect that they are Mary's Sues. A "real" flaw is a flaw that actually makes sense and you see happen. For instance, look at yourself, what are your flaws? As the saying is "Nobody is perfect, and even the most flawless person always has something they're hiding". Like maybe their unorganised, don't know when to shut up, waste money on pointless things, procrastinate heavily, and my all time favourite SLEEP WAY TOO MUCH lol. These are real flaws, if you give characters flaws like these, then readers will be like "Oh, I so feel [Character name's] pain" and they will want to see more of him/her.
All in all, if you have any more trouble , just read other people's fan fiction , like I don't know, Saber's Cross Fate, Dragonfree's Quest, and also The Retelling of Pokemon Colossieum. These stories introduce new characters very well, and those said characters are exceptionally well done in personality and depth.