Personality is a difficult thing to define. If they are talking about good/ bad people, most of us are good who meet all people with good intentions, and that isn't our problem (contrary to what the media tells the world.)
If they are talking about the aspect of personality which really matters, confidence, it is very difficult for incels to develop that without medication. A lifetime of telling yourself you are worthless because women arent interested in you leaves you feeling like an empty vessel of disappointment in every social interaction you have. It is very difficult to charm people when you feel that way, yet alone come across as confident.