I dated a girl named Kristi for over a year, and to this day, no matter how many times I type her name correctly, it will autocorrect to Kristen. A year. A full year.
How do I get android to remember custom additions to its dictionary?
IOS will remember after the third time typing it.
Settings -> Language & Input -> Personal Dictionary looks like what you want, although your keyboard may have its own settings (accessible through Settings -> Language & Input).