Closest would be 甘やかされた which means spoiled or pampered. You could go with the first character 甘 but it is more commonly associated with a basic meaning of "sweet."
More common is to call people selfish instead of spoiled.If that's close enough for you, the word would be:
我が儘
Normally they don't use the second kanji so if you want Japanese people to be able to read it, you'd have to go with:
我がまま
If you're just looking for a single character, you could go with 我 which
carries with it a meaning of "ego" or "selfish".
If this is for a female you could also consider お嬢様 (or 御嬢様) which literally means a daughter of privilege but nowadays is used to say "daughter" or to indicate a pampered (or spoiled) girl. Either in jest or as an insult.