At first glance, I feel like that's kind of a weird question, because what's the point of creating multiple characters if they all act the same...? ^^; I would think some level of uniqueness would be the default choice, not something you have to 'make sure' of...

But then I guess the issue here is that people have different motivations for creating characters...for instance, an amateur might create Character B for the sole purpose of being Character A's best friend, and B's personality or what they add to the story is secondary, if not completely irrelevant.
Or a more advanced (but still misguided) writer might differentiate characters ONLY through character flaws, so as the story goes on and they overcome these flaws, they devolve into a bland, uniform hero hive-mind (* cough* RWBY * cough*).

But I still feel like you could simply...just not do that? ^^; Like, if you have enough emotional intelligence to recognize that people have different personalities, you could just...give your characters different personalities...

Like, let's say Character A is shy and reserved...so let's make their best friend Character B also reserved, but NOT shy. They are as quiet as A (a common thread to build their friendship on!), but only because they prefer not to interact with people, not because of any social anxieties. They are more than willing to take action and defend A when necessary (now A and B have a dynamic!).

On their adventure, they go to a new land and meet Character C...to highlight that this is a novel experience, let's make C really different from the first two and have them be outgoing and loud. Maybe B will dislike C, but the more sensitive A will see their good qualities. Maybe C will tease A at first, prompting them to start standing up for themselves (character development!). The clash of personalities is an opportunity to do a lot of interesting interactions, establish new dynamics, and flesh out our characters.

Lots of stuff will happen, and eventually A, B, and C, will have become friends, and stronger characters as well. But we ain't done yet-- they still need to meet Character D.
Last time we highlighted the new character's personality by giving them something different from the established group: quiet characters encounter a loud one. But now that the established group has more variety, we have to take a different approach if we want to do this again. What do the members of Team ABC all have in common right now...?

Well...they're all nice people. So let's make Character D a mean person!

Now we have a real writing challenge~. The process of Team ABC's friendship may have been eventful, but it could easily be called inevitable since they're all kind, decent people working towards the same goal...no matter what, they'd probably end up seeing eye-to-eye eventually.
Character D, however, may not even want to help ABC at first. Maybe they're even trying to hurt ABC and need to be convinced to switch sides (the classic redemption arc!).

Maybe D's presence in the group will cause friction between ABC as they disagree on how to deal with it-- B might be against even having them around, and C might think they can 'reform' them with enough time. A might not know what to think, and become stressed from watching their oldest friend and newest friend fight and argue.
And of course, D can have thoughts on this, too-- maybe B's distrust offends them and they act out. Maybe B is right and the fact that they're deceiving C makes them feel a little guilty. Maybe they gravitate to A for an 'objective' opinion, and A and D grow together as characters. Maybe all of these things at once. ^^;

TL;DR Just give each new character some noticeable difference from the characters that come before them, and then USE that difference (very important) to develop them all as a cast.

I, for example, have two characters who are both based off pretty much the same part of my own personality a few years back, and I had to take measures to make sure they wouldn't turn out too similar.

I use no other tool but my mind when it comes to designing and creating characters.

I create the story line and setting, first. Filling in the roles needed to tell the story already acts as one big step to creating unique characters with personalities and traits/abilities or contributions that fill out the story elements you need accomplished. The rest will likely happen organically as you begin writing, as trying to plan out too much might make the character-plot relationship stiff.

When I create a cast, I don't really use a tool so much as things like pencils, paper and text documents to flesh them out.

My aim is always to create characters who aren't just interesting individually, but create interesting interactions when you put them together, so I tend to start with a protagonist and then think "who would be a funny or interesting person to put next to this character?"

So with Errant, which originated as a comic called Fan Dan Go back when I was a teenager, I started with Rekki. She's this little, angry, driven, spikey, energetic person themed around the colour red who started life as a sort of "female version of a shounen manga hero". Then I created two characters who balance her to challenge and help her grow: Subo, a big, chilled out, scruffy hippie guy themed around green, and Sarin, a lofty, cautious, intellectual perfectionist themed around blue.

Who do I add to that? Okay, well, those three are all very "what you see is what you get" so let's add somebody a bit more inscrutible to challenge them all! So I added Jules; stealthy, funny, unpredictable, prone to saying inappropriate things themed around purple, and [name redacted], this ethereal, fey, whimsical character with deep connections to the lore, themed around white or rainbow.

If I made the cast now. I'd probably keep it smaller. Like I'm much more likely to work around a trio in my modern comics. But I do really like all those characters. You can pair up any two characters from Errant and get an interesting interaction. That's sort of my litmus test to make sure I haven't made two characters in a cast who are too similar and could be merged into a single entity, or if one of them needs to be changed. If you can't write an interesting scene where any two characters hang out, look into how you can find a contrast between them.

I love building characters, and there's lots of different methods. Sometimes it's just as I feel them. That said, one method that produces really reliably good characters is to start with a central contradiction - two aspects of a person that pull them in opposite directions, create internal conflict, and force you as an author to figure out how this is even possible.

To use an example not in my current book, I have an unfinished book built around a fallen Valkyrie who has spent the last 900 years living on Earth as a convert to Orthodox Judaism (It came to me sitting in a Wagner opera performance during Passover week lol - I was literally munching on my matzah outside listening to hardcore fans talk unprompted about how they didn't think he was all that bad of an anti-Semite.) Anyway, that led to a really hard central question I had to answer - which is how does an immortal being that's literally seen Asgard end up practicing a religion other that the one where she's tangibly part of the pantheon. Figuring out the answer fleshed out not only her character but all the rules of the world building.

Ooh, this is fun. I usually start with a cliche or simple archetype and build a story around them, thinking up a plot where the character's traits or biggest failings are what needs to be overcome by the end and are, in many ways, the actual catalyst of the entire plot. To flesh them out I will add quirks and nuances from my own experiences, people I know and strangers.

For Fields, my lead Jules is a tough vigilante on a righteous quest. She's fiery, daring and tends to hold on to a grudge. I used the "hard on the outside soft on the inside/anti hero with a heart of gold" trope when I created her. Because really, she holds a grudge because she is a sensitive person, who cares deeply about the people she loves and struggles to let go. I also like pairing two opposing traits together. Again, with my main character in Fields: she's fairly quiet and more reserved. Yet she is also very opinionated.

I think mixing and matching those kind of things can help add a lot of depth to characters.

TBH I just create what characters I feel like creating when they're needed.
I start by their role and their story, and then their base personality and looks (these sometimes are the hardest part, but if the characters are friends, family, or share a past in general then I can overlook the existence of many similarities), and then I finally go for the names, etc. Normally I use a name dictionary to make sure the name fits the character's nationality or the nationality of whoever named them, although sometimes their backstory or personality just makes it so nationality isn't a factor anymore, for example, Amy and MC, neither cared about the significance of names so one gave herself the first name she could think of when someone asked what her name was, and the other named himself Molotov Cocktail. And when they're from a fictional country, I just make one up, unless it makes sense for them to have an actual name that exists.

But if you're looking for tools to make characters, I'm sure there are plenty of random character generators in the app store. But don't worry too much about "making every character unique". If the characters have a connection between them, like love, friendship, families ties, etc. then them having similarities between them is totally normal, and can make their connections feel natural.

Another approach you could take is first create whatever characters you want and not worry about similar they are, and then minimise the scale of your universe/multiverse (something you might want to do anyway once you realise that creating a consumable piece of media is a pain in the ass and takes forever :P). You might find yourself 'merging together' characters that are too similar, leaving you with only characters that are different

Not really development but how i make my characters more stand out.

When my character doesn't have strong personality, having foil characters around the protagonist makes them vivid (through contrast) & give different kinds of chemistry. Also character design: gotta have enough differences.

Also when possible, create some less-likely scenarios, it makes you(writer) work out other routes and not following overused routes, something readers will find them interesting.