Ego is my lowest priority: I don't care how cool, annoying, sexy, whatever. It's all about the feeling of playing the character. If I can't play a character intuitively, I won't play them at all. It's something I can feel within 20 minutes of playing a character for the first time.
With that said, in most fighters I can find 2-5 characters that feel intuitive, choosing my main between them breaks down to how effective they are and how well they compensate for my weaknesses as a player and magnify my strengths.
The easiest way to explain this is with the first fighter I took seriously, and the first character I mained: Urien in 3rd Strike. I wanted to play Akuma, but his low health and stun and quarter circle motions and impossible (for me) execution didn't click with me. especially learning a fighter for the first time. So I started exploring the cast. Tried out Necro but he felt weak to me so I couldn't stick with him. Eventually I landed on Urien because I heard that he had unblockables and that intrigued me. His being a charge character helped with my execution barrier and executing his Aegis corner game was easy to implement and highly effective. So what I had was an aggressive character that rewarded for me playing aggressively, who had enough stamina to take a hit when I fucked up, who I could executed his special moves with because of his charge motions, and a killer reward in Aegis setups. As my execution and fighting intelligence refined he cemented himself as my main because of the flexibility of Aegis Reflector - that tool is like Jazz, you can just make it up as you go along.
Looking at MKX, I struggled for a few months with my main because I wanted an aggressive character who also spaced well and counterzoned well to compensate for how much I hate zoning in NRS style games. I tried Cybernetic Kano - he felt great but he felt weak cuz his playstyle requires playing perfectly; a single mistake is too costly in a game like MKX. Tried out Shinnok - he was intuitive but too linear for me. I tried D'Vorah - fun, but hate how she can't deal with pressure or zoning well. Almost gave up, then tried out the most recent DLC character, post-most recent patch, Tremor. With Aftershock, everything I want clicks: he can frustrate the opponent with air Quake, counter zone with air Quake, has crazy buttons, and ridiculous pressure/offense. His armors good, not amazing, but more versatile than most between flash parries and shatter. He feels flexible and I can innovate as I play, using his tools differently and flexibly means I can make it up as I go along, which is my favorite thing.
