With the title, I’m referring in many aspects, playstyle, look, personality, backstory, everything that goes to your head when you pick your main in KI, in my case, that’s Aganos:
I choose him as my last option when I was already done with playing with the rest of the cast (I became interested in the game during the Spinal reveal and Fulgore teaser, later in season 2 with Maya, Riptor and Hisako) I really like his backstory, how he looks, but when I saw matches of him against the cast, he really don’t impress me as much, however, when I start training with him, his normal, special, setups, all the cool things you can do with him, I start doing much better than any other character at the moment (It was the start of season 3) he is a so unique and strong character for me, in the right hands of course, my favorite character in the game and the one I use most and have more fun 
So…how about you???
First thing is character representation, such as their looks, personality and story. The appearance of the character, if they wield some cool weapons and if their moves look cool.
Next is gameplay feel. Do I like how this character plays. I mostly go for rushdown, but I rarely choose charge characters (Orchid in KI1 was an exception for me).
And then there’s if I can pull off some cool combos, how much damage they do, how hard it is to do the combos etc. In KI this isn’t really a problem, as all characters can pull off some cool combos without much effort.
I simply try everyone out in the lab as they’re released. If i have fun finding tech then i play them, if the tech doesn’t interest me then i don’t play them. So far the list of characters I’ve played includes Shisako, Raam, Mira, Rash, Aria, Riptor, and Thunder. Only one character i wanted to play based on looks alone and that was Hisako but once i got my hands on her I was like “nope!” Thank god Shisako exists.
I just give myself a questionnaire:
A) Is it a Battletoad?
there’s no more questions
I choose what appeals to me. This includes looks and personalities and tropes.
Riptor became my number one main because I love reptiles including dinosaurs. Then came her playstyle. Primitive, beastly, violent, swift. This is what I loved about Riptor. That is why I chose her over other characters. Is she a monster? yes! but even she is unique among the monsters, The design of both of her versions are amazing. The first appeared to be a hybrid of a Raptor and Snake (top 2 favorite reptiles) and in this one they added a cyber Dinosaur, that has never happened in fighting games before.
Glacius became my second main because of the power of ice. Ice is my second favorite element and as an added bonus Glacius wasn’t human. But he also earned a spot because he was there for me in KI-2. It didn’t stop the disappointment I had with KI-2 though. He was also an alien that didn’t come to take over earth either.
Sabrewulf became my third main because he was also there for me in KI-2 but Iat that time, I had a Riptor shaped hole in my heart. But I did enjoy his play style in the new KI. I also feel very sorry for him to. Being ridiculed, having difficulty with his mind. Sure, it was his own fault, but yous tillc an’t help but have pity for him.
In no specific order…
Does the character have nice shoes?
Does the character have nice hair?
Is the character Goth?
Does the character have a kitana?
Is the character OP enough to dominate in battle?
Bonus points are rewarded if the character is Goth and carries a kitana. 
For me, looks and gameplay are both important when choosing a main. I loved Shazam in Injustice 1 and Cheetah does a lot of similar things in Injustice 2. However, I don’t like her character/personality as much as Shazam. So I am less ikely to pick her up.
Basically what almost everybody said. I, on the other hand go for skull characters and accessories.
Personality, should not be easy to use, zoning when i need and agressive when i feel, facility to escape or to approach opponents, and overall good cashout.
case i must feel myself inside this characters, i like the feeling to live another life even if its virtual, this characters must represent what iam inside of my deepest mood.
cause a good character help you to been a good main.
with these just ask yourself, who are you , what are your deepest fantasy, what would you like to be in another life . @AlexMark15
me i like to jump , to fly etc… in one word i like aerial characters.so i feel free lol
with the big rooster in KI i think you have more than enough the choice 
It’s funny. As soon as I saw Aganos’ silhouette in the season 2 teaser, I knew he (Broccoli Man) was going to be my main. When I saw his reveal, I was so happy with an incredible-looking giant character with unique combat traits. I can’t say what one thing drew me to him, I just kind of knew.
Connection
I pick Shoto because I want every tool I need. If I loose, it’s my fault, not my character. If I win, it’s my fault, not because I had some advantageous gameplay style.
Maybe I think I chose the character based on looks and gameplay,but in the end I’m starting to feel the character chooses me 
I chose Sadira because I love spiders (not as much as wolves), and her play style just fit like a glove.
I mained jago and glacius because to me they have all the tools I need in a match against the rest of the cast. Also, jago was the first character I got good with in season 1, he’s from the first game (so nostalgia overload) and sometimes I tend to main characters that kind of look like me and jago is probably the closest. While I don’t look like him (lol), glacius is like a transformed version of sub zero who is one of my favorites from MK.
Anything that reminds me of Ryu. Traditional shoto, solid fundementals, overall balanced and fun to play.
I like werewolves. Do I need to explain more?
Yes.
I choose my mains based off of looks first. If a character can’t get me interested just by looking I usually just ignore them.
Secondly comes the characters immediate fun factor. Combos, special traits, moves, playstyle etc. Looks are very important but if whats behind the looks isn’t fun then I’ll just drop the character. However sometimes I miss out on characters who have some fun dirt hidden under their surface play. Sadira is a great example as when I first played as her I didn’t like the always in the air approach she has but once she clicked with me I found her fun to play.
But most importantly to me I find a characters personality to be MOST important to me, whether it be through lore or little gestures the characters makes in game, if a charcater has a personality on display I like then that’ll almost certainly be my main.
I started around when Season 2 launched. She was the only character in the game gameplay and personality wise which definitely set her apart. She walks on screen with a confident grin that she wears until she declares you unworthy prey. She has tons of confidence at all times and I love it.