Bio: A time lord who grew up with their friend the doctor and started to travel with him on every adventure and with every regeneration of the doctor.
The 1st-2nd assistant personality: A mixture of the second and sixth personalities of the doctor/ A mixture of thirteenth and eighth personalities of the doctor.
Reason for regenerating: Was also drained of his life force. (Towards the end of the First incarnation of the doctor)
The 2nd-3rd assistant personality: A mixture of thirteenth and eighth personalities of the doctor/Sarcastic and kind
Reason for regenerating:Was forced to regenerate with the 2nd doctor when they were on trial by the time lords. (At the end of the second incarnation of the doctor)
The 3rd- 4th assistant personality: Sarcastic and kind/A mixture of the 1st and 3rd doctor.
Reason for regenerating:Also died of radiation poisoning but was outside the headquarters of UNIT when he had his regeneration also triggered by Cho-Je
(Towards the beginning of the fourth incarnation of the doctor)
The 4th-5th assistant personality: A mixture of the 1st and 3rd doctor/Cocky and wise.
Reason for regenerating: Was on the last adventure of the 4th doctor and also has a watcher fused with him and regenerated.(At the end of the fourth incarnation of the doctor)
The 5th-6th assistant personality: Cocky and wise/Childish and grumpy.
Reason for regenerating: Also contracted spectrox toxaemia on Androzani Minor.
(At the end of the fifth incarnation of the doctor)
The 6th-7th assistant personality:Childish and grumpy./Dopey and humble.
Reason for regenerating:Either suffering from chronon energy starvation or intentional exposure to focused beams of a radiation lethal to Time Lords (At the end of the sixth incarnation of the doctor)
The 7th-8th personality:Dopey and humble/Charming and loving.
Reason for regenerating: Was also had a clogged vein from a camera; and he anaesthetic that had been given delayed regeneration for several hours.(Doctor Who TV movie)