Kobato.

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Adaption of Clamp's seinen manga serialised in Newtype.

Kobato is sweet and perky girl, but also extremely naive about the world around her. She has a "mission", which is to fill a mysterious bottle of hers with the suffering from people's hearts so her wish to go a certain place that she desires to go can be granted. However, she must not fall in love with a particular boy she works with and whose heart she must heal.

Source: AniDB