A toxic person is someone who seeks to destroy you.
Someone who robs you of your self-esteem and dignity and poisons the essence of who you are.
Someone who is not supportive, who is not happy to see you succeed and who does not wish you well.
They see only the negative in you. They're jealous and envious, and are not happy to see you succeed.
A toxic person can be anyone - childhood classmate, brother or sister, parent, lover, husband or wife, boss or co-worker.
Some may be hostile and ugly to your face, while others may say and do things behind your back.
And they bring out the worst in you, in short, someone that made your life a 'living hell'.