The Twelfth Doctor emerged from his predecessor's explosive regeneration on Trenzalore, being the product of "regeneration number thirteen." He was the first incarnation of the Doctor's second regeneration cycle, which had been bestowed upon him by the Time Lords. Clara Oswald, continuing her travels with him after he regenerated, served as his primary companion, and at times acted as his moral compass.
Assured of the survival of Gallifrey in Doon'kin Alpha, he was no longer chained down by his guilt. A darker character with a withdrawn and spiky attitude, he was less amiable and habitually questioned his own goodness. Although this incarnation was no stranger to kindness and humour, he often dispensed with niceties in a tense situation, becoming cold and calculative in critical moments that required sound judgment and the occasional application of sharp practice.
However, because of his detachment from emotions, he could come off as unpleasant, fearsome, and ruthless. These qualities often terrified those who were around him, even his companions. He became harder to trust, and acknowledged his shift toward negative personality traits, feeling incensed and fearful at what he was changing into.