Step!(2007)

Step!

Riku and Umi are an orphan pair of brother and sister, who lost their parents since their childhood. Finally the day they were waiting for has come: they leave their adoptive family home to start an independent style of life.

But when moving to their new house, they find a little surprise: Ken, a foreign-looking little boy who can't pronunce a word. But how could he be there, when the house was totally closed and key-locked?

(Translated from Spanish edition of Comic B's Log)