Adding to the above, I have found that creating characters on your own is much harder, compared to having someone to discuss them with. Leaving aside the fact that, if you are trying to make a fangame, you should definitely have a team (I am talking from direct experience here, you cannot possibly hope to do everything on your own), this is a general matter: characters are people. And the best way to figure out how people would behave is to talk to actual people. And by this, I don't mean "ask people for advice in creating a character", I literally mean "borrow quirks, verbal tics, idiosyncrasies and whatnot from people you know".
Many authors, specially in anime/manga/jap games (thanks to the fact that Japanese has MUCH more different speech patterns compared to English, making for a more flexible narrative tool) make their characters colorful and easily recognizable by inserting something memorable, if minor, in the way they speak. Allow me to make an example from Fairy Tail, which is my fav anime: let's say that Natsu is facing off against two mooks, that is, two minor characters hired by the current villain to mess with him. Basically, the message the dialogue is trying to get across is simply
Mook 1: Let's beat him down!
Mook 2: Yeah, we will kick his ass for sure!
Natsu: Bring it on!
However, this isn't very creative nor very memorable, is it? So Hiro Mashima adds a little quirk to spice things up:
Mook 1: Let's beat him down, Zatou-niichan!
Mook 2: Yeah, we will kick his ass for sure, Gatou-niichan!
Natsu: Uh... How comes both of you are the older brother?
There. It's the same exact dialogie as before, but while in the first example there was nothing worthy of being remembered, the second one will be remembered by loyal fans as "that one scene with those two guys who were both the older brother".
So yeah, Commander in his post spoke about the substance, here I am talking about the appearance: because sure, the substance is the most important thing, but in narration appearance is important too, the first impression a character leaves on the public is the one that is going to define said character, for better or worse. Which means that making your characters unique (by doing what Commander and the others suggested) is good and all, but it won't be as effective if the public cannot immediately tell that they are indeed unique. So yeah, unfortunately you are making your game in English, which means you have much more limited means at your disposal to achieve this result, compared to Japanese or Italian, but there are still many things you can do: some characters could end every line of theirs with a specific word, some characters could overreact to something very specific, and some characters could insist to be addressed with specific titles or terms.
And your source of inspiration for such quirks should be... Well, pretty much anything: minor characters from anime/manga/games/western comics/movies/books/whatever you like, people you know, or even yourself. That is why I said, it is important to interact with people, in order to create characters, who are also people.