Of course we can know A LOT about Who Jesus was.

We know that "He did all things well"

We know He was the "obedient" Son of Mary, and foster child of St Joseph.

We know that he was perfect man and perfect God.

We know that his "hidden life" was perfectly coherent with his public life.  

We know that He worked well.  

We know He possessed all the human virtues at their perfect state. 

We can use the intellect God gave us to "reason over" and draw inferences from what has been revealed in Scripture. 

We can take all of this to prayer, and ask God to help us fill in the gaps and answer many many questions.

God wants us to know Him....and He gave us many means to do it.