Krishna and Bhishma

Krishna and Bhishm are the iconic personality in Mahabharata. At one place Krishna is considered hero of Mahabharata, Bhishm has also captured great reverence in Indian psyche becuase of Mahabharata. Bhishma has made many sacrifices in his personal life and acted as protector of Dharma and Hastinapur. He even developed special affection with Pandavas. 

On the other hand Krishna grew in rural set up with free mindset and challenging many dogmas adhered by common man in Gokul and eventually he ended up ending the tyranny of Kans by killing him. Later he went along with Pandavas for re-establishment of Dharma with his own interpretation of change. In this way he came in direct conflict with Bhishma and later he became instrumental in killing him. Bhishma has great reverence for him, but because of his own long held belief he was unable to come along with Krishna as he was loyal to Hastinapur.

So Bhishma represent constant while Krishna propagated idea of change.

There is further similarity between Krishna and Bhishma. Both were eighth son of their parents. While in case Krishna, the tyranny Kans killed his brothers, but on the other hand Bhishma's mother Ganga herself killed her sons or better said freed her Vasu from the curse of sage.