The child that takes solace in Zwei in an arc of "Phantom". In another later arc, she becomes Drei, the third Phantom.
Cal's debut in the series is on Episode 11 after an incident that Zwei narrowly survived. She was already orphaned and years later, had been homeless after her big sister had been shot in the head before her very eyes. She escaped from the same fate, but now wants help from Zwei to get revenge against her sister's killers. Zwei was strongly against this, but taught her the nearly same way Ein taught him how to become an assassin.
After Zwei's house is bombed he believes Cal was still in the apartment and was deeply saddened to the point he fled. But it is shown that Cal was out of the apartment and saw the apartment blow up. She waited for 3 days for Reiji to come back but he never did.
Before the "Homeland" arc, Cal (still a child) was unknowingly left behind by Zwei while their secret apartment caught fire. Scythe Master caught Cal in the nick of time and had Cal double-cross Zwei by becoming the Third Phantom.
It is not very clear of how Cal, whom is now called "Drei" aged so fast between episodes 19 and 20, but her skills are much more sharper than before. As the Third Phantom, she now seeks revenge against Zwei for leaving her for dead and Ein for working alongside Zwei secretly.
Cal confronts Reiji and Ein in a church wishing for a fight. Reiji decides to fight Cal alone feeling responsiable for what she has become. Cal pulls out the pocket watch that Reiji gave her when she was a kid, she decided they would start shooting when the clock's music stops. When it does Reiji's shot hits Cal, sadden about this Reiji holds Cal's body as she dies. She tells him her one wish was to be with Reiji until the end, that this final moment with him is all she wanted.