博士

The doctor that only Tsukumo knows his existence ... 

He lost his wife and daughter in the disaster of 1996 and work with Tsukumo for their respectives interests. 
They help each other to reach their goals.

He has many and different memories on Tsukumo but with variations. He remembers a Tsukumo with a yellow bandanna and another with a green bandanna. 
He realizes that their time travel can also be interdimensional, and that he and Tsukumo may not be the first versions of themselves to travel back in time and exceed the dimensional limits.

He then develops his theory on "The complexity of Time" .../