An intense person is someone who takes things very seriously. An intense person could be described as someone very focussed. When something grabs their attention they are less likely distracted. Often, an intense person is described as 'serious' as they are more likely to analyse a subject and see its serious rather than light side. Describing someone as 'intense' is neutral. It will depend on context whether it is being said in a positive or negative manner.

I believe the word intense as an adjective is considered positive. However if the adjective is too intense then it would be considered negative. As a businesswoman she is an intense negotiator. (positive) The heat is too intense for me to enjoy being outside today. (negative)