Place chicken in the slow cooker.
In a separate bowl, combine the sugar, soy sauce, vinegar, ginger, garlic and pepper.
Pour over chicken.
Cover and cook on low for 4 to 5 hours or until chicken is tender.
Remove chicken from the slow cooker.
Skim fat from cooking liquid. (Wow, who knew there is so much fat in chicken thighs! Now I know why they taste so good. This is another reason I need a fat separator, I just used a spoon to skim the fat...)
Place liquid in a saucepan and bring to a boil.
Combine cornstarch and water until smooth, then gradually stir into liquid and stir until sauce is thickened.
To serve, top rice with chicken and cover with the Teriyaki Sauce.