Obviously there was a red bracelet Zauber user that Shizuku adored. And with Shizuku present, she was killed by a blue bracelet Gewehr user which she had defeated _and spared_ before. The parallel to Natsuru is striking, and it gives a strong additional reason for Shizuku to be interested in him.

Shizuku’s motivation is obviously to investigate what’s behind this whole Kämpfer deal, what the Moderators are and what their purpose is. In a way she reminds me of the Claymore character Miria a bit – the strongest veteran Kämpfer of the storyline characters, on a detective mission, and by subtle scheming trying to keep the group alive.

It’s her curse that obviously throughout the whole storyline, Natsuru remains slightly distrustful towards her. I’d say that it’s fairly obvious that she isn’t out to harm Natsuru, just the opposite (now, teasing is an entirely different matter). At the same point of time, she’s probably the one who has the most leeway to pursue him aggressively, exactly BECAUSE of his fear.