B Tome Breaker

Is it not strange that it's not even in the summoning pool? Female Robin is the only unit that has it, and we get... what, 4 copies of her?
Why is that?

I̶'̶m̶ ̶g̶u̶e̶s̶s̶i̶n̶g̶ ̶i̶t̶'̶s̶ ̶b̶e̶c̶a̶u̶s̶e̶ ̶I̶S̶ ̶k̶n̶o̶w̶s̶ ̶a̶b̶o̶u̶t̶ ̶R̶e̶i̶n̶h̶a̶r̶d̶t̶,̶ ̶o̶r̶ ̶m̶a̶y̶b̶e̶ ̶n̶o̶t̶ ̶i̶d̶k̶