Shizuku is described as a girl whose face and voice are rather deadpanned. Her way of speaking lacks the normal voice inflection, so even if she personally didn't intend to, she still came across as being solemn.
Shizuku exhibits kuudere[1] characteristics. When she and Honoka just started hanging out with Tatsuya's group, she only comments from the periphery, but gradually lowered her guard and started expressing herself openly.
She is straightforward, occassionally does merciless snarking, and can say 'mushy' comments out loud without any sense of shame. Because of her usually expressionless face, Tatsuya had sometimes been confused as to whether she was serious or not, but after getting used to it, he realized that she simply doesn't lie. In the worst case, she would simply remain silent.