Bhishma was son of Shantanu and the river goddess Ganges. His mother Ganges had left Shantanu after giving birth to Bhishma. That is different story which would be included in appendix.
Having given his word of honour to his step mother, Satyavati, Bhishma remained celibate all through his life. Marriage was never thought of, nor any sensual desire ever arose in his mind. This great Yogi was adept in many Yogas and practice of meditation.
His truthfulness was such that whatever he spoke came true. He knew every martial art and war tricks. He was the most respected person in the kingdom of Hastinapur. Reverentially he was called Bhishma Pitamaha - Grandsire Bhishma.
He served his step mother, then her son Vichitravirya, later his sons Pandu and Dhritarashtra as a mark of loyalty to the throne of Hastinapur. He was well aware of the weaknesses and ethical decline in the character of his grandsons, and could foresee the cumulative tragedy that was sure to befall the kingdom of Hastinapur. Bhishma was well aware of jealousy of Duryodhana towards Pandavas which he at times openly disapproved. But even this righteous behavior of Bhishma was taken as tilt towards Pandavas and dislike for Kauravas. But, lest his reproach be seen as his disloyalty to Hastinapur, he kept quiet as a silent suffering head of the clan. As a silent and helpless witness Bhishma tolerated all this with melancholy. He never made any attempt to become king of Hastinapur, or to dislodge the weak and morally selfish successor to the throne. Vidura as his best friend understood the peculiar predicament and mental state of Bhishma.
Bhishma, Dronacharya, Vidura, Kripacharya, all righteous in their own way, were still at fault because, though helpless, they served and sided with the non-righteous Duryodhana and Dhritarashtra.