An independent person will not rely on you to look after them or keep them entertained.
Look for someone who can maintain a reasonable level of composure in stressful situations—someone who is not easily irritated by trivial issues.
Take note of how they handle the people in their lives, from their classmates to their parents to the people they meet on a daily basis.
You will both have individual goals, and they will be there to support you. They will not be threatened by the time and energy you devote to accomplishing your objectives since they will have their own.
You want a spouse that will not let people treat them badly, whether they are standing up to you, a family member, or a friend.