I consider these things, not in specific order:
1. Coolness. Do I like how the characters looks, how his moves look or his personality. I could never main a female character, they just aren't cool for me.
2. Execution requirements. Do I have the hands to play the character I like efficiently. My execution is pretty average so I stay away from really execution heavy characters.
3. Popularity. Is the character I like really popular in tournaments and online, or an underrepresented one with potential. I usually go for characters who aren't that popular.
4. Tier placement. Is the character top, mid or low tier. I don't mind playing a high tier character, but I will never again main a bottom 5 character. Ever.
5. Playstyle. What is the characters gameplan, rushdown, zoner, grappler, setups ect. I can pretty much adapt to play any style but I always gravitate towards grapplers.
For reference I main Lobo in Injustice, Adon in SF4 and probably going to main Kano in MKX.